NNCP (http://www.nncpgo.org/) can be helpful here as well, in conjunction with the SQLite session extension.
Especially the combination of Syncthing and `nncp-xfer` is super useful. NNCP usually has strict routing which can be a hassle to guarantee (machine D only gets packets from machine A if they went through machine B and C first).
Using Syncthing as a central storage for NNCP packets circumvents the strict routing requirement.
It sounds like litestream (https://litestream.io/) might be useful to you here
Re:The Session Extension
by unstatedAnswers on Monday December 13, 2021 @07:15PM
(#29546531)
> without network access
I don’t think it’ll be useful here
SQLite never ceases to amaze me.