I'm in the same boat. I use Cloudflare email routing to route mail for my domain to Gmail. That covers the inbound email, CF routing provides a catch-all option and you can direct individual addresses to different inboxes.
For outbound, just use any provider that gives SMTP for custom domain. I used Zoho for a while, recently went back to running my own server for outbound. In Gmail web interface, you can add other addresses to send mail as using external SMTP servers.
All of this is of course not very good privacy-wise. Both CF and Google can read your mail... But putting that aside, the setup works really well. You can get your custom domain to Gmail with the price of a cheap email service. Zoho is around 10€ / year, but you could even use something like Amazon SES, I understand with low volumes it's practically free.
I thought about just forwarding from my own MX to Gmail, but that may cause problems if spam gets forwarded. SPF + DKIM setup is simple for traditional use, but forwarding all mail requires the original headers included in the fw mails, seemed like CF probably knows how to handle that better than me.