Yes, you don’t need to transfer the DNS to Mythic to be able to use their DNS servers.
If you really want to continue updating domains on BytemarkDNS, you can copy over the existing upload script (or a freshly downloaded new copy) from an existing server - you should be able to push updates from anywhere, just not create new accounts.
OctoDNS integration (and a re-write of sympl-dns) is next on the list once I finish getting sympl-ssl rewritten, at which point support for a range of DNS platform API’s will be built in (with you only needing to specify which one and the relevant keys), at which point automatic wildcard certs will be possible!
I believe I have an up to date static mirror of it’s state not long before it was taken down - it needs some cleaning up and search functionality adding, but I should be able to get it up and running soon.