I've found Cloudflare is cheaper than Google Domains, and allows for more flexibility of setups. A $12 domain on google can be $9 or less on Cloudflare. (this is a per-year price. So the cheaper you get the domain, will exponentially be cheaper over 10+ years)

Cloudflare is also a layer of protection to your pihole. You shouldn't run in to that many issues by buying a domain but I'm not sure what you're trying to do. Just buying a domain is like buying a username, it doesn't do much until you point it somewhere. So if you're pointing it to your pi-hole to host something then I'd look in to hosting services to save yourself a bit of a security issue.

What is it you're trying to do?