I asked a question and forgot about this thread, apologies for that.
If you were playing a battlemaster fighter and still felt you were one dimensional then I think you are going to want to play as some kind of caster. A Warlock/Paladin multiclass might be best if you still want to be a main damage dealer while getting more options in how you play.
I don't have a save where I could try the fight against Zrell as you'd have experienced it so I tried attacking her in the tower. I put away my best equipment, set it to tactician and played with Lae'zel, Wyll, Shadowheart and my character who I'd respecced as a sorcerer. I was level 8 at this point so I reset the party to fairly basic builds at level 7, except for Lae'zel who I took Great Weapon Master for as her level 6 feat.
First I had Shadowheart cast Hold Person on Z'rell to bait her counterspell (I think a cantrip would do here, but just to be sure) then twincast Haste on Lae'zel and Wyll. Shadowheart then cast Sanctuary on my sorcerer. Lae'zel proceded to tear Z'rell apart in short order, I used Precision attack followed by trip attack, then enabled great weapon master and used the rest of her superiority dice on precision attacks. Her and Wyll killed Z'rell in a single round that way and proceeded to take the rest of the enemies on that floor apart.
I don't know if that's helpful to your building as a fighter, as a sorcerer, how to get past that encounter next time or generally. My group would be stronger with Lae'zel and Karlach rather than Lae'zel and Wyll if that gives you any ideas for group composition, e.g. playing yourself as the fighter, respeccing Gale as a sorcerer and taking Karlach so you get to be the damage.
