Two weapon fighting is definitely a little weak compared to PAM + GWM (which basically does its job with much better damage). You have to be careful when re-balancing it though since it's pretty easy to create something OP.
I like your method, but personally I'd try to balance it via feats. There's two ways I would do it:
The first would be to introduce a "Power Attack"-style feat (like GWM) for Two Weapon Fighting, perhaps something like a -3/+6 instead of GWM's -5/+10. I'm not a huge fan of this method though TBH since it treads on GWM's identity.
The second way would be to try and adapt 3.5's Improved Two Weapon Fighting/Greater Two Weapon Fighting feats to 5e. Essentially, the way I'd imagine it would go is something like this:
Feat Name: Improved Two Weapon Fighting.
Prerequisite: Extra Attack class feature.
- When you use a bonus action to perform an offhand melee attack, you can make an additional melee attack.
- Any additional attacks are treated as offhand attacks and follow the same rules as them.
- You can select this feat multiple times. Each time you do, you gain an additional attack. In order to do so you must have the appropriate number of "Extra Attacks".
- You cannot select this feat if the resulting number of offhand attacks would be greater than the number of main-hand attacks your character can perform with a single action.
This homebrew feat would work decently well in tabletop. It would be powerful but still fairly balanced since at most you'd be able to do 3 offhand attacks in total (and only if you're atleast a level 12 fighter) and would require spending atleast two feats to do so. It would still be weaker than GWM + PAM in terms of pure damage, but it would also be more consistent than the former since it doesn't have the -5 to-hit penalty. Essentially it would just be letting you apply a more limited version of the extra attack feature to offhand attacks. I also specifically limit it to melee attacks in there just so that it doesn't make dual hand crossbows with crossbow expert + Sharpshooter even more powerful. If it's still too powerful, you could also add in a -2 to-hit penalty on extra offhand attacks.
The issue with adding a feat like this though would be Larian's homebrew would make it massively OP. More specifically being able to get more than one bonus action per round would also allow you to get way more attacks than you should. A 15th level character (Fighter 12/Rogue 3) would be able to attack 12 times per round, every round, which is ridiculously OP.
