I'm not sure if there's a real cap, but there's a natural cap just from the weapon you can slot the essence in. At lvl 90, stats go up to +9 and the special skill(infliction, twilight etc) up to... +6? I forgot but you can see the max stats on the weapon. You can keep upgrading essences until you reach that. Past a point it's a pity game, the odds are very very low to get a raw proc, but upgrade enough time and you'll get the powder things to guarantee an upgrade, so it's just a numbers game until you get a maxed essence. The gains are obviously barely worth it but if you have nothing else to do with your stamina it's a good dump.
On the spending, I spent 1$ to grab Endmin's weap, and I'm considering spending 10$ to buy the battlepass IF 1.1 info looks interesting but for now I'm just holding on to it. I've been at the "nothing to do" stage for a while already, I just log in, do daillies, log out, other than events, which is fine, but pull economy looks pretty dreadful currently so I'm waiting to see what they say about 1.1(which won't be for a while, I'd expect 1-2weeks before the patch as the usual timeline) to decide if it's worth spending more time and money or not. I haven't bought the sub cause honestly it feels like shit value in this game and I don't feel like rewarding that kind of practice. Unless pull economy is really low, the 5$ for ~14pulls is just horrible compared to some other games. And if pull economy is low enough that 14pulls is a significant boost, game's not worth playing F2P unless they switch to a 1 char per patch release cycle(very unlikely with them already teasing 6 chars in the livestream though).