desec.io can be used with any domain registrar and has an API with support for various ddns clients (ddclient, lego).
deSEC is a free DNS hosting service, designed with security in mind.
Running on open-source software and supported by SSE, deSEC is free for everyone to use.
Edit: To clarify, desec.io does not sell/rent domains. Desec has to be set as the authoritative nameserver on the registrar, then desec can manage domain records instead of the registrar (which usually also provides their own domain hosting for “free” by default).