The Signal Desktop app you are trying to install must be somehow outdated, the desktop apps I use are working perfectly (both GNU/Linux and macOS) and I never encountered this issue. Maybe try to get some help from the community? https://community.signalusers.org/
Please, Synapse isn’t production ready, at all. See this issue for example: https://github.com/matrix-org/synapse/issues/3364, reported back in 2018, labelled highest priority since then and still not fixed. This is indeed a huge problem, because if you don’t have shit tons of money to put in your database server’s storage you’ll quickly get stuck. Some workarounds mentioned are non-safe hacks and shouldn’t be necessary for a production ready messaging server.
That’s now clear to me that recommending people to buy the Librem 5, a 700$ phone that cannot pass a call after 3 years of development, isn’t a good idea. Do you think it’ll eventually be fixed?
Regarding Scaleway lack of rDNS for IPv6: it's because they don't support it. Customers (like I was) asked for it many times since 2016 but it seems too hard to implement for them.