I use both samba and NFS (with freebsd) on the same disk for my freebsd, linux and win7 desktop machine. All works perfectly fine.

I’m running an rsync daemon for large transfers and use copyparty for easily grabbing files on phones etc.

https://projectsandcastle.org/status it worked on sandcastle, maybe it hasnt been ported to mainline yet?

if usb works then usb networking may be a better option

You have an iPhone 9? It doesn’t exist, you probably have something else.

iPhone 7 got Linux ported, so yes you can. Try looking for postmarketOS (though there aren’t that many working features, it’s good enough for running servers)

i use seamonkey mail. it literally just works.