If it's an rpi serving 400 req/s, yeah you're probably right. If we're talking more about maintenance by a handful of people and a few hundred internal users, PHP is fine and an rpi can handle that load just fine.

What minimal alternatives would you suggest given the constraints?

As mentioned in a sibling comment of mine, there are some alternatives:

- Elixir's Nerves: https://nerves-project.org - Golang: + https://github.com/tinygo-org/tinygo + https://gobot.io/ + https://github.com/golang/go/wiki/GoArm - Rust I suppose you can just directly compile for any SoC like the RPi.