I put in a few hours on this game because I have been playing SC2FMFR and I saw that it was by the same creator. It's funny because it's essentially focusing on the thing I hate most about SC2FMFR (chasing around the random drops) but this game is a lot more successful because it's the focus and it's not random and distracting. It's sort of meditative in a way, even; I found myself getting into short states of flow every once in a while.
But, the main thing I ended up disliking was shuffling between the upgrade menus, since it messes with the aforementioned flow when I have to go juice up my upgrades and spend water coins. I feel like it might be neat to try having upgrades and rain on the same screen, to try to promote more flow and less menu flipping? Menu flipping is the bane of a lot of incremental games tbh, it's a subtle thing that ends up messing with flow a lot, at least it does for me. It's far from a dealbreaker obviously, but I'm big on flow, and for an actively oriented game like this I think it's more important to establish and maintain flow.
Additionally, a mechanic like golden cookies from Cookie Clicker might be a fun thing to add too, starting a little minigame with higher intensity, or something like that. It DOES get pretty rote, even for someone with a lot of patience like me. It does feel like your framework is good here, focusing on just the one mechanic rather than several that coalesce into something chaotic. It just needs a little bit MORE chaos right now, in my opinion, something to keep it from feeling like it's not quite so repetitive. But not too chaotic... it's a hard balance.
(I did also notice that the best raingold ratio is achieved by pumping both your level and your stockpile of rain high. Only having one or the other high is less good. I don't really like that personally; I'd prefer that the rain spent on upgrades and kept in stock boost raingold linearly. This is a minor complaint though, it's just a bit of a noob/nonobservant person trap.)
So, overall, I think there's good potential here but I mostly just find it a bit tiresome and not that fun currently. I think it's because the ratio of activity to variety is a bit whack currently; active incremental games need more variety to work, while idle games can work with less.