how good is the RF 50 1.8 STM ?

How sharp do you need it to be (what is your display size and medium)? Would you want to shoot it wide open or stopped down? I like mine. It's small, light, cheap, and plenty sharp enough for what I use it for. But what I use it for is mostly low light shots of people, so I don't need or want ultimate sharpness to the corners. It's pretty sharp at least as far as the rule of thirds points even wide open, and very sharp by about F2.2. For the price, I'd say it's an excellent lens. But if you want almost perfect sharpness right across the frame wide open, you're probably going to have to pay a lot more money. The RF 50L is supposed to be amazing, but big and heavy and almost $2000 more expensive. You could also adapt the Sigma 50 F1.4 Art EF version. But that would also be a lot more expensive (about $800 more) and much much bigger and heavier. If you are only using manual focus, there are probably quite a few sharp manual focus lenses that you can use on an R with an adapter, that also won't break the bank. My advice, though, is to get the RF 50 F1.8 and try it out. It might be fine for you, and if it isn't, you can sell it without losing too much money (it's so cheap in the first place), or even return it within the return period.

Edit: I see you live in the Netherlands. I don't know how the prices of the lenses I mentioned compare for you. I'm sure the RF 50L is still a lot more expensive than the F1.8 version, though, and I would think the Sigma Art is a lot more expensive too.