The cloud flare sync doesn't work that way, automatically. Each customer would need to setup their domain on cloud flare, create an API key, add the key to their enhance account, activate the sync in their enhance dns settings. The default config doesn't turn on cloud flare protection for dns entries either, so they would need to edit their @ and www record to turn on the proxy. If you're not setting this up for each customer, then you'll want to create some detailed instructions pages to help guide them how they can setup their own dns using cloudflare or other provider.
IMO it's better to keep email services separated, just because it's such a time sink and so many issues will come up about it. We've used a cpanel email node for years where we have dedicated email services available to clients - but we highly urge ppl to buy google workspaces if they need professional email that comes with support.
A lot of ppl nowadays just straight up don't offer email service at all, I totally get it too, it's a big waste of time and $, and the struggles with it never end - IPs getting blacklisted, ppl needing their hands held to setup a mail client on their phone, complaints about missing emails, etc etc etc... It's exhausting.
