Welcome to Sailfish OS and the Forum!
There’s a discussion about X10iii vs XZ2 here:
I’d say if you want to start now use what you have and familiarize yourself with the OS, its app stores and life with Sailfish.
Just be aware that performance (speed/responsiveness) is better on newer devices but otherwise everything is the same.
Don’t focus too much on running Android apps, you have other devices for apps you think you need.
Check the status of 2G/3G/LTE of your network provider - if your device of choice does not have VoLTE support and your network closed down 2/3G already you’re not going to have much fun with telephony.
About X10iv, v or vi support no-one will be able to give a good estimate, support depends in part on Sony releasing fixes, and in part on Jolla’s resources which are sparse and likely focussed on other areas. If I’d have to make a guess I’d say “at any time in 2025”.