Use proper 5e stats and XP rewards, revamp some of the encounters a bit (and I don't mean a complete overhaul - just tweaks), give us party of 6 as a standard norm, and you'd have balance. You would not need a level cap.
As usualy you just trade one problem for another. :-/
Exping would then be awfully slow ...
Just try to imagine you would need to suffer level 1 during whole crypt, attack on grove, fighting the goblins in secret passage, fighting the harpies, Blighted Village, and maybe something more.

And there is another ... strong encounters.
Imagine you play as a Gith ... and your character really agrees with Laezel ... then imagine you would only get to level 3 during whoooole surface ... and then you met Githyanki Patrol. :-/
What would you do?
And remember this is suppose to be ROLEPLAYING game ... so there is no "i just go to Underdark to get some more XP". :-/
Encounters:
1. 3 Imps = 600 XP divided by 2 (you and Lae'zel) = 300 XP. Level up! Oh wait! We're using lame baby Imps that aren't real Imps with real Imp abilities and Resistance. OK. So let's cut that in a 3rd. 100 XP each character? Party of 4 would be 75 each. Party of 6 = 50 each.
2. Repeat on deck. Optional fight. 3 More imps. 50-100 XP. Wow! I'm almost at Level up time depending on party size.
3. Intellect Devourer and Thralls. Optional fight. OK. Severely nerfed devourer. Normally they'd give 450 XP. Let's really nerf that because dang that's a lot and it's a baby Intellect Devourer without Resistance, Devour Intellect, Full Health or Body Thief or MultiAttack. So, let's say 100 XP split between 2. 50 more + Thralls... maybe 75 XP. So, I could be REALLY close to Level 2 already just from 1 necessary fight and 2 optionals.
Not exactly slow leveling up, if they gave more appropriate XP.
4. Second Intellect Devourer. Optional fight. Another 50 each character? Yeah. Now they'd level up if only MC and Lae'zel. Party of 4 + Lae'zel might be close to level up but not quite there.
5. Helm. Yeah. 2 Imps and a Hellsboar + another 2 Imps and a Hellsboar + maybe killing Zalk... and dang if you're really good you could kill 2 cambions also? Definite Level 2 by this point. 4 Imps = 800 XP. But, again, we're dealing with severely nerfed Imps, so let's say a 3rd XP. I'll round to 270 total for the party. Party of 2 would be roughly 133 XP, party of 4 is 67 XP (roughly), and party of 6 is 45. That's just the imps. Add 2 Hellsboars and maybe add an additional 30 XP to the total to make it an even 300... and that's being kinda nerfed XP-wise. I won't even add in Zalk or the cambions.
Yeah. I'd say in all fairness, after everything you go through, whether you are a party of 2, 4 or 6, or anything in between, if XP was done even remotely well, you'd be level 2 easily by the time you get off the Nautiloid.
Which!!! would make fighting 3 actual Intellect Devourers do-able if they went with a party of 6 as a standard and allowed you to make 4 customs right at the beginning. Then you could do party of 4 customs + Shadowheart against 3 actual devourers with real stats. That would be doable if all 5 party members were at level 2 by that point.
But, regardless, let's say they don't make them real devourers and keep them baby mode devourers. Let's say they also base it on a party of 2, just you and SH. That's still an easier first encounter after the prologue at Level 2, which could easily be achieved by that point.
But then, don't hand out XP like candy, like others have said. Next XP dished out should be if you fight the 3 fishermen, or you kill Astarion. XP should be low because they aren't very difficult encounters. Maybe 50 total for the entire party for the fishermen as an entire encounter. So, party of 2 = 25 XP each. Party of 4 = 13 and party of 6 = 9 XP. Now you ain't leveling up super fast regardless of party size because XP is being divided out amongst party members. Thus, instead of needing a cap, you are gaining XP based on party size. The smaller the party, the more XP you get. However, the challenges are going to be harder if you are actually succeeding with a smaller party size.
Gimblebock and Mari and Barton and all the mercenaries? Assuming something similar to Bandit Stats, 25 XP per Bandit for the entire party. Maybe 450 for Gimble, Mari and Barton, provided they actually made them tougher and not just one of the crew. If just nerfed, like they are currently, they should all dish out 25 XP per person you take out. Gimble, Mari, Barton would be 1350 XP altogether if not nerfed for the entire party. Their minions: 2 outside, 1 at the door, and 3 others, that's a total of 6. So, if not nerfed, that's a total of only 150 XP for all mercs besides the 3 leaders. Keep the leaders nerfed as they are now, and that'd only be another 75 XP, for a total of 225 XP for killing all mercs at the Dank Crypt. 225 divided by 2 is 113 each, divided by 4 is 57, and divided by 6 is 38. Again, not a whole lot.
So you get to Level 2 by the end of prologue and would be at level 2 through Dank Crypt at least (unless you did kill Zalk and got tons of experience for that and were at Level 3 by the time of the Dank Crypt. Either way, gaining baby experience through to the grove is not going to really level you up to level 4 at this point).
Goblins = 50 XP per goblin for the whole party. However, one thing to also keep in mind is that as a DM, I would not award full 50 XP per goblin on the battlefield if I also have a ton of NPC's helping the players beat them. So, even if at the grove fight you consider all goblins and the bugbear and the worg for XP, you shouldn't award more than 700 XP-ish for the entire party to divide up - but as I said, I'd even reduce that severely since there is a lot of help taking down a goblin captain and a single crew of 4 goblins + 1 bugbear and 1 worg. I'd probably only award a third of that - half at most. So maybe 300 or 350 for the entire encounter at most. Divide that by 2 = 150-ish for party of 2, 75 per character for party of 4, and 50 XP per for party of 6.
Level 1 is 0 XP to 299
Level 2 is 300 XP to 899
Level 3 is 900 XP to 2699
Level 4 is 2700 XP to 6499
Level 5 is 6500 XP to a whopping 13,999
Level 6 is 14,000 XP to an even more whopping 22,999
As you can see, depending on how they award XP, and party size, and the enemies you are really up against, they could easily control how fast or slow you level up throughout the entire EA, making it so that even if you do the entire EA, you'd only be roughly Level 5 or 6 at most. It just depends on how they handle it.
And if you didn't do the entire EA, and you went for the jugular, if you enter an area that's harder than you should be travelling through, that's on YOU. That's how open worlds work. You might be able to skip to the Underdark, but you should know that it's a VERY dangerous place, and you might actually not want to skip to it until you are at least Level 5 or 6.
Either way, if done right, a cap isn't necessary. The amount of XP it takes to get to the next level at higher levels caps you enough.
Even if they change NO XP reward values in the game, as they currently have it, you'd be naturally capped during EA at level 5, possibly only getting to level 6, if you did EVERYTHING in EA - and that's a pretty big IF to get to level 6 by the end. There's your cap. Don't severely limit players by capping them at a certain level that they've worked hard to earn. That's BAD DMing. Real bad. My players would be furious if I did that.
Bah! But here I am again wasting my time on this. They're not likely going to listen to me, so what's the point? Everything they're doing in this game is extreme, so they're not likely going to chill now, are they? They'll level you up super fast to Level 4 and then cap you, making you go through the entire Underdark/Cursed Land, Grymforge and Moonrise Towers before allowing you to level up again? And then what? Jump you to 6 right away?
Yeah. Makes no sense. None at all. I'm sorry. Capping at 4 during EA makes sense because you may not have ALL the versatile elements ready for the game yet. You may have to work more on spells like Fireball and Lightning Bolt and Fly and such. So, capping at 4 during EA to keep things more chill and manageable until you're ready to uncap is smart. If they cap you at Level 4 AFTER EA, and you can't be higher than 4 during the remainder of the EA, that's REALLY bad. Not just a little. REALLY REALLY bad.
Dang! And there I go again. Gotta give this up like a bad habit. You really know how to suck me back in Ragnarok. 😠