Party size is gonna be a tough one.
Almost all party based RPGs struggle with it, almost all old DnD games had the option of not taking a full party, usualyl "Blanacing" it by not having to share your loot.
Usually you could make this work, but only with very gimmicky builds.
The same is true for OS, so i think youll have the option, but imo its hard to make a game in which the "Intended feeling" works with having less or more party members.
On casters:
Stahl i think you are beeing a bit optimistic here. The problem with class balance with DnD is not that wizards are stronger than martials in "pvp" which isnt realy a thing. the problem is that Wizards ofthen are better than martials at what martials are supposed to do.
The Knock spell is one of the usual things brought up.
The squishyness of WIzards is basically irrelevant as casters dont solve combat by damage but by just making the targets go away in other ways. Wizards ofthen have options to solve problems with a spell. Object hidden behidn traps? just teleport it here.
Big demon to fight? just banish him to a pocket dimension. Starving in a desert? just summon some food.
Limited spell slots usually do the opposit actually.
Instead of limiting the caster, they mostly mean that the party has to rest more ofthen or else the wizard is useless again. You would need to seriously restrict resting options to balance casters this way.
I agree with you that just buffing fighter damage output (which is very high in almost all DnD editions anyway) isnt the solution.
The solution is to give martials more options to do in combat.
5E has a class called the Battlemaster, i think that the battlemaster maneuvers should be standard for all martial classes, why would a barbarian not know how to sweep someones leg.
