Just transfer it to whoever wants to take it over

https://www.digitalconnectmag.com/how-to-transfer-domain-name-ownership-to-someone-else/

FreeDNS (afraid dot org) allows anyone to create subdomains under your domain for free, so could be a good fit?

An alternative would just be to have the selfhoster message you the IP and you update the records manually...

Or even ask the selfhoster to setup a DuckDNS and you add a CNAME record for that, which would allow the selfhoster to update records themselves on duckdns