If you just want the custom domain, Fastmail is fantastic. Rock stable, fast, 2FA, modern UI (as good as Gmail, but fewer annoyances), great IMAP, very flexible in terms of domains and rules (uses Sieve), reasonably priced.
Moving to Fastmail from Gmail was a massive improvement for me. Being able to quickly hook up custom domains to the same inbox is a big boon. Plus it costs less.
I recently migrated all my domains to fastmail (left my legacy gapps account as is, just in case) and am super happy with it. I am using a regular google account on the side for all the googley things I need but haven't opened gmail ever since.
The only thing I miss are labels. Once you get used to how to use them, it's hard to go back. I migrated my mailing rules to use folders but that forces me to check every folder for a new mail and don't have it aggregated in my normal inbox.
I am a super heavy email user and live on Fastmail every day.
I'd just love a boomerang/snooze feature.
And agree on labels, however, it's just due to them following their own open source standards and with IMAP/JMAP emails can only live in one place at a time.
One thing I don't like is that their Android client which supports push notifications (a feature I personally valued in Gmail) also has some weird UX.
For example, the message actions bar within the "view message" pane. At first, I wasn't aware it existed. I thought you had to archive/delete messages from the inbox by swiping it! I only discovered it by accident when my finger missed a link in an email. It seems like it appears when you tap somewhere in the message itself, and it appears over the top of the bar that holds the message title and the back arrow.
Very silly in that regard, but otherwise quite solid. Their servers, service, and desktop offering are great and I found their pricing quite reasonable.
And btw, on iOS/iPhone Fastmail does push notifications using the native email client: https://blog.fastmail.com/2015/07/17/push-email-now-availabl...