Going with two storage tiers is an interesting option for heavy io workload. I’ll take this into consideration.

IMHO with netcup you get a lot for your bucks. I’m running a RS2000 and I’m running a mail server on it. However, this was quite some work as some IP ranges may be burnt by spammers and you might need to deal with some DNS blocking lists to get yours removed. In the end it works fine if you do all the stuff with DKIM,DMARC, SPF etc. and have IPv6 configured for your MTA.