There's a backup function. You can set it to run at certain intervals. I included it in my syncthing sync so I have a backup of my whole signal whenever I run syncthing.
https://support.signal.org/hc/en-us/articles/360007059752-Ba...
https://github.com/signalapp/Signal-iOS/issues/905
…but there's been no apparent progress.
On some core usability & functionality matters, Signal appears to be a really, really, lethargic development team, both before and after the arrival in early 2018 of $50 million in funding.