Unfortunately I don't remember about the retention setting. I would avoid Pidgin since I'm not so sure they implement fully omemo encryption. Try Gajim.
Ah good to know about Conduit!Well that's how the protocol works saving data on every participating server. Simplex chat might be another alternative but is on early development as far as groups and desktop client.
Many years ago I used XMPP with Prosody and it was ok. You can setup some retention time for attachments and history depending on the space you have.There are clients for multiple platforms but not very much advanced vs competition. Security is good with omemo protocol. In so e clients you have to turn it on since it's not by default. Audio video calls on one to one is good as long as you setup stun coturn server. Android to iOS calls still in progress. Check out Snikket for an all in one solution.You might want to also consider Matrix (dendrite option ?) for your needs.
I found Manjaro to work perfectly on my MacBook pro 2013 and recognized immediately the graphics card. Flawless experience so far.
Cantata ftw !!