No, it's not particularly important to me that the OS is FOSS (it's a bonus I appreciate, but what I dislike about my work Mac is that it's hard to configure and changes between versions and bundles crapware, etc., not that it's not FOSS).

Another Unix would be ok as long as it's not hidden away under a forced UI, with some things unconfigurable, required, etc.

But ultimately I want to share dotfiles between my desktop and phone (sure I might have to accept some amount of conditional guards..) - and that pretty much means Linux and ideally Arch. But only because that's what I happen to use on non-phones.