If you don’t need super fancy graphics and don’t mind some walls of text, both of Owlcat’s Pathfinder games, Kingmaker and Wrath of the Righteous, are absurdly intricate character builders. There are probably 20 classes, close to 100 subclasses? You can generally mix and match them as you wish other than not being able to take two sublasses of the same class.

Both games also include a roguelike mode DLC that can be played separate from the main campaign, which are mostly combat and great for testing new builds.

Do yourself a favor and install the mods that let you define your standard buff set and cast it as a macro. Otherwise you’re gonna spend a lot of time manual buffing between fights.

KM buffbot: https://www.nexusmods.com/pathfinderkingmaker/mods/119

WOTR bubblebuffs: https://www.nexusmods.com/pathfinderwrathoftherighteous/mods/195

one thing i’ll add is that i’ve heard good things about guild wars 1, rift, and ff11 but have struggled to break into them. the only MMO that’s ever really stuck with me is wow, and i play that super casually now. i had fun on ascension (customizable class private server) for a week or two but the classic experience doesn’t grip me like it used to.

You already named Baldur’s Gate 3, and I don’t think I’ve found one more interesting or elegant than D&D 5e. Maybe other folks are more well-traveled than I am, but I’ve played most of your examples, and 5e still takes the cake. You can still do better, especially since not every attribute is equally important across classes, but it still makes for fun synergies as is.

I really enjoyed the previous big RPGs by larian, Divinity Original sin 1/2.

Apart from the gameplay just being fantastic, it has a very open class system where you put points into different abilities and you can freely mix and match whatever you want.

It also puts a huge emphasis on environmental hazards and how they interact, which makes the combat feel very novel.

Not an expert on RPG but I liked Bravely Default a lot in that regard because rather than multi class you kinda collected classes and switching them on the go made the gameplay so much more varied.

I loved Wildlands; not necessarily the multiclassing, but the fact it was just Borderlands in a fantasy setting, and they made it work. Really well, imo.

A game I like that has multiclassing is Kingdoms of Amalur. It’s pretty arcade-y and kinda simple, but it’s also just really fun to play.

Istaria may be old and a bit obscure but it is a personal favorite of mine. It’s a small mmorpg with many playable races (and playable dragons!) that features a multiclassing system. A single humanoid character can basically level any class(es) and mix and match the abilities for a custom build.