Do you use an ESP32? Could you? You could use ESPHome https://esphome.io/

Ideally the device can create a wifi hotspot when it isn't setup that I can join and setup all the needed config settings. WLED does this as well https://kno.wled.ge/

I'm not saying necessarily without a backend. Just being able to "self-service" via API (ie if everything I can do via the app, I can do via curl, it could be interesting).

Self-hostable backend (benefits: privacy; less trust required; I know it'll still be usable even if you shut down your backend in 5y) would of course be great but a separate thing to the above.