Hi! Sorry for not replying earlier!
A little update: The ammonia kept going down and algae started to grow (probably diatoms? it's insane how quickly they grow!). Nitrites also came down a little later. I also did a small water change to get the nitrates down a bit as well.
So the current situation: nitrites and ammonia went down to 0 last week and the rest of the parameters also seemed safe, so two days ago I got my first coral (pulsing Xenia) and a small (dwarf, I guess) red tip hermit crab (the LFS suggested just one for a start to prevent fighting. hope it'll be OK like that). The coral seems to do fine, however I'm still trying to find the right flow for it. It's not pulsing very obviously now, but when I turn the pumps off I can see that it is still moving, so I hope it's fine. It does look big and better than when I first put it into the tank so I guess that's a good sign. The hermit crab still seems a little shy. He's been hiding inside the (wet) RRR most of the time now. He seems a little more active at night though (that's when I've seen him walk around a little). Hope he'll be fine as well! He is really small though, even for a dwarf, so I wouldn't be surprised if it'd be molting any time soon. Did put some small (but bigger) shells in the tank for him of course, and hoping to add a few more soon. So I think I just have to leave it be and keep a little eye on it to make sure it is kind of okay (for as far as I can see)...
I didn't try the ammonia adding method though. As I kind of felt a little unsure about the ammonia solution I have...
Sorry! Not sure if there's anything else I could try or test that would also be of any help?
Anyways, for now I just hope my two additions will survive and things will stay kind of stable!