You can change the app.yml now. You’ll need to wait until the release to run another command line upgrade. It’s not clear how many more beta releases there will be before a new stable is released.
Stable means fewer updates, but not necessarily fewer bugs. It requires a bit more expertise to run stable than tests passed. Critical security updates do get included in stable, but ux issues are likely to remain broken until the next stable release. Third party plugins are less likely to work with the stable branch as it’s less well tested.
If you want fewer updates you can just update less frequently.