Dude, you cannot have it all. Yeah, rainbows for everyone all the time would be nice, but amount of customization and simplicity are conflicting design goals.
Since you obviously dislike optimization, you probably haven't considered the meta-state of character optimization. Lemme break it down for you:
Character customization can be handled in two basic ways inside a class-based system:
1. Having a relatively small number of very broad classes where two characters of the same class can perform very differently.
This approach requires that players make a lot of decisions after they choose their class. (In terms of D&D these decisions include ability score allocation, race, feat selection, skill point allocation, background selection, which version of various class abilities to choose, what weapons to use, which spells to memorize, etc). Taken to the extreme, this is a point-buy system where there is only one class and it's all about the option selection.
This does seem a pain in the posterier compared to oldskool roll stats, pick class, pick race, pick weapon, paint yer mini chargen - but in return for all the tacked on complexity the system offers a much greater degree of character customization.
2. Having a large number of very narrow classes where all members of the same class perform very much alike. In this version, players choose a class and that's the only big decision they make - everything else is folded into that class. However to allow for each of the players to play a character type who appeals to them and who isn't overlapping completely with another player's characters you need a very large number of classes.
Taken to the extreme this is something like an arcade fighting game
Pick one and press start to complete chargen
Except that in an arcade fighter if you can't figger a character out, you can wait 99 seconds to plunk more quarters and try another - in a TTRPG you're stuck with the character for at least the session if not longer, so some players are gonna want to think about it beforehand. Yet, in order to make an informed decision and figure which of the narrow classes best fits their own playstyle, a player a player has to read through all of the available classes. Except that it takes a LARGE number of such classes to allow a similar degree of customization to the system of few but broad classes.
As a minor digression, the system you imply where customization is handled via multiclassing is a subset of this system - such a game just allows people to mix and match levels between classes rather than writing each potential combination out as a unique class.
So either there are few classes to pick between but many things to pick within each class, or there are very few decisions other than class but many classes to pick from - in neither case is it a trivial optimization problem.
Not unless you intentionally limit the amount of decisions involved in chargen by reducing the amount of customization available between characters.
And while there is certainly room to limit customization while still leaving interesting playspaces to explore in many TTRPG systems, that can be a hard sell to many players and does increase the odds that two players will play characters that greatly overlap.
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