Thanks. What in a case when for example new game comes out which I would like to play, but it requires newer firmware. Is there a way to avoid it?
In short, some system version requirements can be bypassed, but others cannot be bypassed. You should keep your system on the latest system version that your CFW supports. The latest version right now is 11.0.1, and Atmosphere supports 11.0.1.
If you're installing games using Tinfoil, you can bypass soft version requirements, but this won't always work if the game used a relatively new SDK.
BTW. to not mess with backups of nands, I have access to CFW sysnand and since I do have all my games basically on cartridges and only one game backed up on SD, how can I prioritize booting with primary fuse to my Sysnand CFW? If I'm able to do that, then I can easily reinstall that one game without backing up anything
First, make sure you backup any important save files on your emuMMC using JKSV. You can also use JKSV to restore those save files on your sysMMC.
After that, you should remove the emuMMC partition on your SD card and expand the partition to encompass the whole SD card. That will free up approximately 32GB on your SD card. You should also delete the emuMMC's Nintendo folder on your SD card, which may free up more space.
As for your question, I'm not sure what you're asking. By default, fusee-primary will boot CFW on your sysMMC.