I am sure this will break the second time you run Windows Update.
Although, since UWP doesn’t work it’s a moot point, that means you probably can’t run Windows Update.
Then you are confidently wrong, because Windows Update rarely (if ever) touches the bootloader, while this is the main innovation of the WinBTRFS project.
I am sure your custom bootloader will not be overwritten. All types of hell can happen, borking the entire installation -- and your bootloader will still be perfectly fine.
For the rest, there could be references, but it's just lingering legends now: yes, in the past, it could happen BUT only for the Windows partitions (recovery 2700, efi EF00, msr 0701 c: 0700) and only if your partition was too small for a specific update. Then windows would do some careful resizing, in the worst case creating another partition out of its C: to avoid touching the rest. So it would't break anything, just like how ntfsresize works on linux!
For the selection from the UEFI, it was only if you used the default bootx64.efi and tried to repair the boot (windows detected this is not normal and would replace it by something to chainload bootmgfw.efi with)
Anyway, since Windows 10 it doesn't happen, and I never had any problem at all with 11.
Just use the recommended partition size (or more, I regularly make my EFI 4Gb), name your bootloader something else than bootx64.efi and it will be fine.
The most windows might do is to reorder your UEFI boot entries (then go to settings and you can boot another system, same in recovery: just select your linux UKI or grub EFI)