I only played Stellaris off and on, but I went years without buying an expansion and always thought the new systems were complete and better than what they replaced when I returned. Breaking current saves is frustrating, so I guess you would need to delay an update if you had one you planned on returning to.

If you didn't know, you can roll back to older versions of steam games with some work. A few games have a built-in system, but most of the tile you have to manually replace files after redownloading the old versions.