I haven’t used Signal.
Is your ‘registered Signal number’ your phone handset number?
From this page:
https://support.signal.org/hc/en-us/articles/360007318691-Register-a-phone-number
[You are right, Delta Chat uses AutoCrypt, which is OpenPGP based.]
A simple network share.
Does your router have a USB port?
Plug in a thumb drive (32-64gb is fine) and configure it in your router’s admin interface to share on the network. (FTP and SMB shares)
An easy way to have all devices in a house able to access a shared directory. (Pictures, movies, music, files)