I really don't know, how you come to the conclusion, you need a fighter in the group. Lae'zel is the most underused companion in my games, because I don't need her. I have beaten the game several times with spellcasters and rogues and never had a problem. Gale and Shadowheart are nearly always in my group (and they don't miss often) and then Wyll or Astarion. I don't normally play fighter, so I'm more often than not another spellcaster or a rogue. Works well for me. In fact BG3 might be one of the few games, where I don't feel bad about not having a fighter in the group. Different pkaystyles, I guess.
You don't need a fighter, you HANDICAP yourself if you don't take one, both because Frightening Attack is by far the most reliable and powerful CC in the game against bosses like Bulette, but also the sheer damage burst of action surge and the innate survivability. Same goes for rogues.
Of course, if you're using rogues, you're using a class even more broken than fighters, because dual wield surprise attack with consumables and fire dipping can down any lv5 boss in a single rotation.
But it is indisputable that in terms of output, mobility, and CC, the martial classes are far better than spellcasters currently.
And when you tell me your casters don't miss alongside Shadowheart, give me a video of your sacred flames in an entire playthrough or even just against Bulette or Githyanki patrol.
Hell, I'll even give you leeway and ask you to take screenshot of your hit percentages of Shadowheart/Casters against Githyanki compared to Lazael and Astarion backstab.