There should be a time when Alpha 20 is available exclusively as an opt-in experimental version. This would be the time to go to the game’s properties in your Steam library, Betas tab, and select A19.3 or whatever version you want to keep playing. Then you’ll stay on that version indefinitely, even after A20 goes stable and everyone else downloads it be default.
I’m sorry to read you lost a huge base going from A18 to A19. However, these days nothing should be lost unless you try to play a version X saved game while your copy of the game is version Y. And the game is supposed to prevent you from doing that now: incompatible saves will be marked with a red version number and the Start button will be disabled.