Hi!
I think the problems you are experiencing (and they are typical problems that we have predicted while designing the game) arise from the fact you are approaching the game as an arcade game, a shmup, whereas at heart we intend it to be an RPG style progression game that merely looks and feels like a shmup. It's a really weird concept and we totally predict to lose a lot of potential players like this - like yourself probably - but that is the way we intended it (and luckily we are not here to make a quick buck

we wanted to try something new and tricky and unintuitive).
To tackle your points individually, with this in mind: The bullet power and bullet spread upgrades each become individually more expensive as they increase, seeing as they can drastically increase your firepower by stacking together - so obviously only focusing on one of them would make no sense (as they stack). If you only increase bullet power without bullet spread, you will find that it becomes unreasonably priced very soon; if you do the same with shot spread, you will find that one to become unreasonably expensive. As each bullet becomes more powerful with the bullet power upgrade, and the shot spread upgrade gives you more bullets, it's clear that the two upgrades are optimized when they are upgraded side by side. So instead of selling the one that you found to be more expensive, the simple solution in your case would be to buy the other corresponding upgrade, which will then be to the benefit of both these mutually stacking upgrades. (To put it in RPG terms, you can compare the two dimensions to hit rapidity and damage per hit in a traditional RPG.) Maybe that's all very obvious to you; I'm not sure if I took your point correctly, but hopefully that explains that part?
Sticking with the theme of RPG - the cash of the game, again, you can consider as the space shmup version of XP points. Seems totally weird, right? Like a space ship casting spells. But so as you gain XP in RPGs, it becomes harder and harder to gain more XP; the same goes for the cash in Cosmonator - as you gain more ranks you get less and less cash, due to the rank modifier. So when you go back to do levels (including side missions) that you didn't do earlier, depending on how powerful you are, the level will give you less reward. Same as if you went to a previously unexplored area in an RPG when you are strong already: it will be easier for you, but you will get less reward, because now you are higher level. In Cosmonator, the earlier you tackle the side missions, a) the harder they will be for you obviously, because you have less firepower, but b) the more reward you will get. That is not a bug - it is actually one of the fundamental elements of the game. It probably seems really weird if you are considering this in terms of simple one dimensional shmups like Raiden, but again, if you put your mind in the world of RPG progression, it seems much more intuitive. (But yea, I gotta admit, the way we did it is really weird. I don't blame you for finding it disorientating.)
Also, in terms of selling stuff - again, in RPGs, if this was XP, it would seem totally OK not to be able to sell, right? In most RPGs you cannot sell abilities in order to get XP back. Honestly, though, it is something we have considered; at least giving like a 50 percent cash back option or something. At this point I feel like being stuck with something once you buy it feels right with the game, even though I know many people will be frustrated by it.
Have you ever played Dark Souls? I think in order to understand what we are trying to do with Cosmonator, you should think of it not in terms of a perfectly smooth, happy user experience (like many shmups try to do), but a challenging, alienating, mystifying, and slightly frustrating one. I know that's really weird for a shump to try to be. And unfortunately, obviously it will cost us a lot of players, of good reviews, and potential cash in the bank. But more importantly we hope to find the few key players who are into what we're trying to do, and who will appreciate it

perhaps you are not in that group (or maybe you are, and you just need some time to get into the vibe! after all, the game really only starts to make sense later on, when all the different complex upgrades and advanced spells start to combine, so the demo is a very limited way to experience it), and that's totally cool. Probably most people will agree with you, alas. In any case I still really appreciate all the time you took with the feedback, and I'm happy you engaged with the game. I would say: stick with it! Buy the full version! But I'm a bit reluctant at this point. There are so many excellent user-friendly shumps out there that I think you will enjoy much more, I just gotta be honest
