Hmm, CIFS document provider on fdroid might work?

set up a local VPN on your NAS/file server and add your phone to the VPN. Next, map a local drive on your phone to the NAS. Dunno if you can do that on iOS, but at least on android you can. Disable modify and delete permissions on the NAS folder, so that the phone can only write/read from it, then use a camera app that lets you choose the save location and set it to record video to the network drive. Requires a pretty solid connection, but is extremely robust.

Probably wherever the trees are exploding