Are there any FMP like low code products that would also allow user sign up/sign in? I believe the problem with Access/FMP were that they were single user or trusted user setups.
FMP supports multi-user setups. You can even use external auth servers including Google login now.
I don't know if you can create a "new user" flow in your app, but you can add/register users and assign them to permissions groups from the FMP admin dialog.
Surprisingly you can pay to have your DB hosted on AWS. That said, it's very expensive.