I recently started using rtx and I’m a big fan.
Works for a long list of tools and makes it very easy to remove lots of install commands and just do “rtx install” in any directory to install and switch to right versions of a set of tools. Also works great on CI using same configuration files.
Nix is interesting and fine if you are a solo developer with time to learn the details. I did try it briefly but find the learning curve of rtx is near zero - it does less than Nix but is very easy to adopt in a team. Compatible with asdf but faster and easier in some way, and actively developing.
Buying a domain that includes email would probably work, or just getting a less commonly used email service. Fastmail is good and has many alternative domains.
Have these services actually turned you down? Usually this is a marketing thing where they prefer work email on forms to assess commercial opportunity, but I’d be surprised if they refuse a personal email.