As a solopreneur myself for the last 3 years, I’ve tried every solution mentioned here.

The best solution I have found and what I currently use is https://www.outseta.com/

It handles everything: from authentication to help desk, to transactional emails, marketing emails to sales funnel, subscription management.

Everything is basically no code so you don’t end up needing to build anything yourself that isn’t central to your value prop.

You can get it running in one day. Highly recommend.