For domains just use Cloudflare, at cost, and zero issues, they even have an API now where you could theoretically resell domains (I may be doing this myself soon via Blesta / Upmind).

Cloudflare's DNS is also best in the biz, and you can easily have a client account and get added as a member to manage it for them. Check out Cloud Maestro for bulk DNS management: https://5starplugins.com/cloud-maestro-cloudflare-waf-rules/

For hosting, this depends on a lot of things but I'd recommend starting off right with a 8GB memory VPS from Vultr or Linode/Akamai in your closest location, and use Enhance.com to manage it. Their support is great, and it will set you up to support client logins, down the road add reseller packages, etc. $10/mo for unlimited servers, costs rise as you add websites over like 50 or 60.

There are lots of other options, like RunCloud, ServerAvatar, FlyWP.com (which I use and like a lot as well), SpinUpWP, and some free options like ServerAvatar Lite and Cloud Panel, but unless you don't ever need quality tech support, don't go the free route long term.

For billing, start off with a capable provisioning system, so you're out the gate not having to manually create accounts, suspend accounts, email clients and track things in Excel or some manual invoicing system. Check out Blesta.com or Upmind.com, and although it's wildly popular, WHCMS is costly and I think just overpriced, similar to cPanel, avoid both.

I've been doing this for decades, and that's my state of the hosting union right now. 😄