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Yes, iFlash is the best IMO, though a bit on the expensive side.
The tutorial can be found either on YouTube or iFixit.
iPod 5th gen (aka Video), flash modded and with Rockbox.
If going without Rockbox, Rhythmbox on Linux can upload mp3-s to the stock iPod but (IIRC) not flacs.
Hetzner has a service called “Storage share”, which is just Nextcloud server that they manage for you for ~5€ per month. They notify every update by email and the sever’s 1TB.