I have 6 out of 20 achievements in Microsoft Solitaire Collection (Win 8):-
Can I continue this in the Android app or is that an entirely different game?
Different game. The one with cross play is under UWP, not Win 8
Just an FYI you can set the date back to days you missed for challenges and it will add to the months progress. If you had pro in a month the bonus for that given month should still apply.
I might go back to this sometime, when I started it I think it only had 3 achievements.
The problem will be as I have read that this game is not backward compatible with Windows 10, I would have to reinstall Windows 8.1 which may be too much hassle.