Most people don't realize, but you can actually send email from a custom domain with a free Gmail account.[0] You have to go through a 5-minute process for each sender address, but I used it fine for ~10 years.

I recently switched to Fastmail, and one of the features I like about them is that they support wildcard sender aliases, so if I sign up for a service with someservice@mydomain.com, when I hit reply to any emails to that address, it automatically sends from someservice@mydomain.com even though I've never specifically registered that identity with Fastmail.

[0] https://thedigitalnonprofit.com/use-gmail-own-domain-free/