Uninstalling updates to the Play Services app will do nothing to the operating system.
I don't understand what you are saying about going to Play Services via settings and clicking on details to go to the Play Store, but if I want to open the Play Store I just open the Play Store. And I admit I know nothing about AR stickers (no interest whatsoever to me). Of course the apkmirror Play Services is a beta, so it's possible it contains bugs.
I don't think there should be any long term downside to uninstalling updates to Play Services. In the short term you'd have an older version, which may have some effects, but it should update again in due course. Uninstalling updates will clear data for Google Play Services as well though. Again that won't have long term effects, but could mean that you don't get notifications from some apps for a while (GMail for example - I cleared Play Services data a couple of months ago and GMail's new message notifications stopped for several days). So maybe try clearing the apps cache first and see whether that helps, rather than going straight to uninstalling updates.