On the 3 dots menu, theres “configure plugins” (or smthing like that), then you have to enable something like “expose filesystem” (dont know the exact wording because mine is not in english).

You can also share access to your phones entire filesystem with kde connect, so you can browse you phones storage from dolphin as if ot was connected through usb and copy entire folders to/from you phone.

Maybe kubuntu has some weird firewall default settings. When i tried using opensuse some years ago, it took me quite some time to figure out that it was its firewall that wasnt letting me use my printer and some other stuff i cant remember

I have an instance on my phone running 24/7 which does the bridge. But i dont use much storage, i mainly work with text files, so the pc at work syncs with my phone, and when i get home my own pc gets the files from my phone immediatly. Its been working really well for years for me.

Its good, i started using it because it allows you to inverts the colors, so you can turn your pdfs into white text on black background.

Theres SiYuan that is open source and is in the same spirit as notion, obsidian,… (regarding usability)