If you look at the spells I just pulled from a few of my players, the differences in spells aren't really in damage. I circled the things in purple that are consistently different. Now, Clerics and Warlock (limited spells) are the truest casters in the game. The wizard needs some work. You talk about big damage but the wizards really don't have it because if you are talking % of total enemy health per round relative to a 5e game, they are doing significantly less than your normal caster due to the nature of the encounter. Spiritual weapon, Tashas, Toll the dead, Spike growth, Spare the dying, Absorb elements, SHIELD, and sanctuary are spells that drastically change not only the DPR, but the shutdown for movement (spike growth), and advantage (tashas). This is a difficult conversation to have considering some of the most commonly chosen and used spells are missing from the book. Casters doing damage is nice, but there are other ways to guarantee SOME damage - spiritual weapon and spike growth especially are crazy good to just throw out there and now you have a spell that is pretty much guaranteed to succeed. Things like Tasha's are such a staple because of the prone condition that they have to *continue to save on* (without making an ice patch your own team can eat shit on).
And the point, again, is combined with HP bloat that line of "Take out a single enemy" is no longer an accurate statement.
If you want to say BG3 EA current selection of utility spells is a bit lacking I can agree with that, but the wizard class causing big amounts of damage is in the game. Stuff like scortching ray (especially when coupled with hex, and yes, you can have both as a wizard) does ton of damage for example (read 40+ danage). Stuff like thunderwave when correctly applied (i.e. when you position yourself so you can throw several opponents down a cliff...) is deadly. No martial character can hope do do the same in early game (barring lucky criticals and potions of speed which are way more unreliable than casting a spell with right positioning) or to give the party the same utility. So yes you can easily take down single enemies of even groups of them with spells in BG3 EA but it requires you to plan your moves carefully, luring them where you can hurt them the most. Also... Tasha is in the game already.
Cant sleep in the middle of a 7 round fight, now sure where you are getting Null & Void from.
Yes but you can sleep right afterwards and recharge everything, have another encounter, go nova again, then sleep and so on, that way your caster doesn't need to manage resources at all. As a DM I can tell you that resource management and recomended number of encounters per day are basically the of the main ways used to ballance caster classes compared to martial classes.
Half the time "tactics" means climbing up a rope or a ladder, burning it so the melee cant get to you and hiding out of LOS and just dumping arrows and firebolts and then dodging out of LOS again until they die. I wouldn't call it particularly clever, its holing up repeatedly so you don't get burned down and you save your spells for when you need it instead of using spells because they might work. The round economy breaks the 3 round fight and spell slot usage because of how long it takes to decrease enemy DPR from HP bloat. So, what do you do, you give as much advantage to martials as you can because they're the ones who are going to consistently hit and do damage. Its way better to empower them than blow 3 2nd level MMs for 12 damage on a creature that has 25hp instead of 10. Now you'll knock them down with ice and let that martial go to town. Thats the economy change that starts coming down the pipe because of the matrix. Again, this game does not play like D&D. You dont use the same spells in the same way for the same reason. You position martials and casters differently than you would. You are playing on a retreating action consistently and its dropping the "5e players are impossibly strong" theme that DMs run into time and time again. 5e favors the player significantly, and this is definitely not.
I destroyed a ladder the grand total of once (in the Zhent hideout to be precise). The most bloated and unfun fights I had in BG3 were in the goblin camp after I killed the 3 leaders and that's not because it was a difficult fight. It was actually pretty easy as I used my casters and archers to get rid of lower hp enemies (a lot of them with 12 hps so killing them in one hit with spells or sneak attack was not a problem), the problem was the time it took for the thing to end when the outcome was never really in doubt. As for your final lines... you are making a lot of assumptions about D&D. Yes, PCs are supposed to be powerful but depending on the group playstyle and level of experience they are not guaranteed to just walk around winning every fight, I for one don't DM the game that way although I also try to refrain from killing PCs due to sheer bad luck (on the other hand I'm the kind of DM who believes actions have consequences, if you do something utterly stupid in one of my games you are probably going to pay a price for it). If all I threw my players were easy encounters designed to make them feel great about themselves they would just ask me for more challenging encounters. And again the recomended number of encounters in D&D 5th edition is 6-8 encounters per day!
Edit: forgot to mention that currently wizards can learn any spell in the game, cleric spells included. This is wrong and needs to change imo but it certainly gives wizards a lot of utility they don't usually have at their disposal, especially not early on.