Could do. I’m going to assume you’re already aware of most popular action RPGs in the style of Borderlands or Diablo, and that their skill trees are nowhere near complicated enough for you (or else you wouldn’t be asking!), so I won’t waste your time with recs along those lines. I’m also excluding MMOs, since those are deliberately structured so that it takes forever to actually work your way up their skill trees. Let’s see what else might fit the bill.
The first game that springs to mind is CrossCode. Not only does it have huge, sprawling skill trees with all sorts of complicated interactions and branching paths, but every time you finish a major dungeon, you gain access to a new, entirely separate skill grid themed after that dungeon’s element. You can swap skill grids at any time, which enables some truly disgusting tricks, like changing to a completely different build right in the middle of performing a combo. You don’t have to exploit the mechanics in that sort of depth – the game’s quite happy to just let you muddle through, at least on the lower difficulty settings – but it really rewards the time and effort you put into it.
Second, if you don’t mind console ports, you might have a look at Tales of Vesperia. It uses the “learn skills from your equipment” approach you see in many JRPGs rather than the skill trees more common to Western RPGs, so it’s not 100% what you’re describing; on the plus side. there are rather a lot of them – each character has in the neighbourhood of 120 skills they can pick up over the course of the game, and you can freely respec your mastered skills any time you want, so there’s a lot of territory to cover build-wise, and it definitely qualifies as an action RPG.
Finally, If your rubric for “action RPG” covers Metroidvanias, you might have a look at Sundered. Its particular brand of pseudo-roguelike gameplay is admittedly something of an acquired taste, but if light puzzle platforming interspersed with getting swarmed by literally hundreds of monsters is your idea of a fun time, you could do worse! Beautiful animations, too. As for the skill tree mechanics… well:
What you’re looking at here is about half of what your skill tree looks like on a 100% clear playthrough; I don’t have an exact count at my fingertips, but there are at least a couple hundred nodes, and the relationships between them are more than a little baroque.