Specifically the backend. I'd love to hear your reasons. Do you keep one eye on what the stack would be post MVP?
1. Old schoolish (VPS - Maybe DO, Django/Flask/Rails/Remix/Next with postgres)
2. Supabase etc with JS/TS on either side of the network
3. Lambda/Cloud functions with Firebase/Dyanamo DB/Cosmos DB etc
4..n. What else and why?
Early on I would still have to lean on Alpine heavily for various JS interactions but with all the features Liveview has been adding and improving on (Hello JS module!) I find myself needing it less and less. Liveview really has been a game changer for me. Tailwind has more or less fixed most of my frustrations with CSS and has worked itself nicely into the company design system resulting in nice re-usable components that can be easily customized for those one-offs. Elixir is a nice language to use and I find myself missing features of it when working with other languages. Phoenix is well structured for projects and the newer generators solve a lot of common issues/features in projects like user auth in a reasonable way.
Depending on the needs of the app I'd look into deploying to Fly.io otherwise I've been using Kubernetes with little issue. I've toyed with Nomad for orchestration but haven't used it enough to give an opinion. I do wander if it's a nice middle ground for those who don't need the full suite of what K8s offers.