I'm not sure I understand which clients and which services you use here. You use Fastmail, which is a paid service for email, right? But it sounds like you also use Zoho services as well. Why both?
Zoho is so cheap: I pay $12/yr for multiple domains and multiple email addresses on each domain. It was a little tricky to validate the domains and get them added, but it's been smooth sailing ever since.
Should I add Fastmail in the mix? What does that add here?