Moving off cloud services to lower-cost provider like Hetzner, Vultr and DigitalOcean might provide a lot of cost savings.
I also imagine they're using managed SMS services from one of these clouds, and moving off them to a combination of local SMS gateways in each country can also further reduce costs (and in one case I've personally observed, by upto two orders of magnitude). This obviously pushes a lot of complexity on Signal's side, but is usually worth it.