What are criteria to be a good alternative? I personally work for inbox.eu for the last 10 years

We are a small team and have experience with providing email services for almost 30 years.

Our management also is lately very pro AI and we are reevaluating how we will move forward with developing features

I concluded that the best alternative is to use a simple email host like iCloud with your own domain, which is free if you pay for their storage, and then move everything important to Signal.