without knowing anything about domain.de, if you own it, how many other certificates are issued for it … I can’t say.
What is best depends on what you use domain.selfhost.eu for.
Is this mainly a personal domain for you ( maybe access to your home server, or a system only you and a few others are likely to use, like your own owncloud ) ? If so, I’d just get a domain for that. Or do you have a domain that you control, that you could use a subdomain from ?
If it’s something a lot of other people use - it depends how well domain.selfhost.eu and if you mind changing the domain name. Since you are using a subdomain of someone else’s I assume it wouldn’t be too much of an issue to move to a new domain, but I don’t know.
If I give you an example of mine, it may be easier. I own the domain serverco.com but I want to have a URL (with cert) for my home, so I use housename.serverco.com as the URL, with DNS through cloudflare, and then use the ddns option on cloudflare to update the A record IP address whenever it changes.
I have also ( to test) obtained a free domain name ( from freenom.com ), cloudflare DNS ( I’ve also tested with cloudns.net, which also offers free DNS and ddns API … there are many others to ) … to update the A record to my IP address as it changes.
Both of the above methods mean I have control over the domain name and DNS - so there is no issue with obtaining certs.