I haven't used Signal in a while but can users select their own servers/nodes/relays or whatever you call it or are users locked in on one? Maybe you rotate them if they are down?

Picking nostr:npub1exv22uulqnmlluszc4yk92jhs2e5ajcs6mu3t00a6avzjcalj9csm7d828 as an example, sure there are predefined node operators and of course they can be down but everybody can run their own node as well and use that instead.