We're reaching the end of an era wherein billions of dollars of investor money was shovelled into tech startups to build large user-bases, and now those companies (now monoliths) are beginning to constrict their user-bases and squeeze for every single penny they can possibly extract. Fair or not.
Now more than ever, it's important for us to step back and reconsider whether we want to be billboards for these companies anymore.
For anyone unfamiliar, some good resources to have when starting your degoogling journey are below:
Privacy Guides - A list of privacy-respecting services you can use.
Plexus - A crowdsourced information bank of service compatibility with degoogled devices.
This random PDF - A study from 2018 detailing data that Google tracks about its' users.
You can use a privacy-respecting mail service and use any mail app you want with it. K9 Mail app is pretty well regarded, and there's no shortage of decent mail services, some suggestions:
https://european-alternatives.eu/category/email-providers