Honestly if you were able to live without a phone until now, living without Gmail (or what you call a mainstream email) is pretty easy.
There are still plenty of email providers left and right that will allows you to subscribe without a phone number. You’ll probably have to pay though. What may be harder in this precise use case is getting a domain name (which is not mandatory but really helpful if you want to be able to switch providers easily) but I’m pretty sure you can still register domains with a landline number.
I've been maintaining my own mail server for almost 20 years. The thing though, I found that sometimes I end up needing to use one of the bigger email providers like gmail, apple mail, etc because otherwise my mail is classified as spam.
There are still plenty of email providers left and right that will allows you to subscribe without a phone number. You’ll probably have to pay though. What may be harder in this precise use case is getting a domain name (which is not mandatory but really helpful if you want to be able to switch providers easily) but I’m pretty sure you can still register domains with a landline number.