I have a setup with separated dns and domain since 2021. Using a CSK with unlimited lifetime, I never had to rotate. And could easily also migrate both parts (having a copy of the key material)
Register only has public material
The master is bind9, and any semi-trusted provider can be used as slave/redundency/cdn getting zonetransfers including the RRsigs
> Using a CSK with unlimited lifetime
Well in cases where I have had to deal with DNSSEC, I've had to rotate the KSK annually for compliance reasons.