> I've heard people suggest you should split registrar and dns solution, but I don't really understand why that would be best practice.
Simplifies migration. If your DNS records are tied to the registrar, and you need to move the record (maybe selling or moving to another registrar), then you can run into a problem where the DNS records are not accessible while the name is being transferred. Not an issue if the nameserver record points somewhere else.
Anecdotally, many registrars I have worked with (including Namecheap/GoDaddy) have terrible DNS management consoles/APIs, and limited options for access control. I have also had issues with certain TLDs not being available to move to better registrars, though I'm not sure if that is still an issue. Either way, moving DNS to a separate, standard provider definitely makes things easier to manage, especially if you are working with a lot of domains across different registrars.