I'm using Posteo and have absolutely no problem with it. The base price is 1€/month and for my purposes I haven't needed to buy any extra stuff (like extra space or aliases). It also allows access via mail clients by IMAP and POP, which is something I've seen many popular gmail alternatives not providing, despite being IMO a pretty important feature.

You mentioned storage space is important for you. The default size is 2.0 GB, but you can acquire more by paying +0,25€/month for every extra gigabyte up to a maximum total of 20 GB.

It should be noted however that it is a German company and therefore has to comply with German laws.

The one thing I'm not really convinced by is their approach to spam. The web interface doesn't provide any way to define rules to filter out spam except for filter exceptions, but the service already filters out spam for you and it will never reach your inbox. I would normally think that's a bad idea, but I've never received any spam nor have I noticed any mail going missing (except for my lemmy.ml registration mail which I remember I had problems with but I don't remember if it was Posteo's fault and if yes if it was their spam filter in which case it could be allowed to reach your inbox by adding it as an exception).