Is it somehow unthinkable to you that a powerful barbarian might have found some way around charm spells/other single targetsave or sucks?

Idk, did he ever learn to read?

You can certanly balance the combat, don't get me worng, but I think that a party from level 11 onwards can outmanouver a Berserker in ways he es not able to keep up. Most of the casters should fly by that point and its been a while but I think by level 11 casters already have access to some way to have the action economy on their side, like Animate object or summon animals (is that the druid's one?). In the minwhile the other party members can spam save or suck spells as you said! O guess the real breakpoint is level 13 when they have access to forcecage.

I really wish martial characters had some superhuman "spells" like your clasic super hero shockwave or those big jumps... I know monks can jump really far, but I wish most characters could!