I use the free plan from Zoho and it's been solid. But... I have been using it for a while and it is my understanding that certain features I have (IMAP) are no longer in the free plan.

Just looked at their pricing and it looks quite sane. How good is their calendar and does it sync to iOS and Android?

Their Calendar is exactly like any other caldav icalendar, works fine on iOS and any other platform using the calendar standard protocol.

Been using Zoho for many years, switched from my free grandfathered Google Apps account to Zoho around 2014.

I use the Zoho app (iOS and Android) for mail and calendar, so yeah, it syncs that way. I haven't tried setting up another mobile app.

Cloudflare Email Routing is still in private beta. I'm eagerly awaiting it

It's open now: https://blog.cloudflare.com/email-routing-open-beta/

One thing that wasn't immediately clear from the blog posts --- does Cloudflare's solution allow configuration to also send emails from the custom domain email address?

In GMail (and using Google Domains to host my DNS) I've configured this by inputting various settings as per the guide here:

https://support.google.com/mail/answer/22370?hl=en

The key relevant step is: "For school or work accounts, enter the SMTP server (for example, smtp.gmail.com or smtp.yourschool.edu) and the username and password on that account."

Is that kind of thing supported with Cloudflare?

Thanks. I've been on the waiting list for quite a while. But I didn't get any notification.

You just have to apply for it for each domain you've added and they approve it within a few days