Selfhosted

Many paid email services such as Mailbox.org or Protonmail allow you to use custom domains. You don't need gmail.

I picked up a black friday deal from mxroute for $15/year. Unlimited domains and 300 email/hour. I have it setup as the relay for my internal postfix/dovecot setup. It also hosts one email account so if I mess up my own configuration I can still do email.

Looks like their current deal is a $5/year for 1 domain https://account.mxroute.com/order/micro You could still set that one domain up as a relay.

I've used SimpleLogin for awhile and it's worked very well for me, everything shows up and everyone receives their mail as expected, very happy with it. AnonAddy is another alternative, although I personally haven't tested that one. They're both pretty cheap and recommended by PrivacyGuides. SimpleLogin was purchased about a year back by Proton. Who run Proton VPN, ProtonMail and a few other products iirc. https://www.privacyguides.org/en/email/#email-aliasing-services

Check into Google workspaces, it’s a paid service now but I got a family email set up 20 years ago and it’s awesome. You get all the access to Google stuff, administration, there’s plugins and stuff it’s just a Google account with a cooler email and an admin panel really.

I use Zoho Mail to host my email these days; I like their UI a lot.

Google Workspaces can do exactly this. You set up custom domains for it to handle mail for, and you can set up aliases or whatever so you aren't making/paying for more than $6/user/month (assuming you are ok with the limitations of their lowest plan).

If you use cloudflare for your DNS you can take advantage of their email routing feature to handle incoming mail on your domains, and can apparently even jump through some hoops to set up a normal gmail account to be able to send email from that same account by just changing the from address when composing an email. I have used email routing for some things, but not gone through the outgoing email workaround.

For sending I am using smtp2go. It's free if you send less than 1,000 emails a month.
For receiving my dns host provides free email forwarding

Microsoft offers custom email domains for Office365 family plans, but looks like they're closing that to new customers in November

https://support.microsoft.com/en-us/office/get-a-personalized-email-address-in-microsoft-365-75416a58-b225-4c02-8c07-8979403b427b

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.

Check into Google workspaces, it’s a paid service now but I got a family email set up 20 years ago and it’s awesome. You get all the access to Google stuff, administration, there’s plugins and stuff it’s just a Google account with a cooler email and an admin panel really.

Read about Google workspaces

I use namecheap’s email service privateemail since a couple of years. Google workspace was great, but don’t need the other features anymore and this is fifth of the price https://www.namecheap.com/hosting/email/

Purelymail.com sound great for your use case, it's also very cheap.

I use mailbox.org - they recently changed the pricing and now you'll have to pay a minimum of €3/month to have your custom domain.

IIUC apple has an email offer at $1/month that allows a (or many? idk) custom domain and wildcard mailbox.

Honestly, I am considering doing the opposite and starting to self host my email... I loathe comparing the differences between various plans/tiers of the various providers and with commercial email one is bound to do that once in a while (plus €3/month is kinda steep... I mean... not that it's a very high price per se, but I pay some €6/year for a certified email mailbox and it's kinda strange to pay much more for a "normal" one)