It's just software, you can install it on any server. No need to limit yourself to (currently?) overpriced Pis.
There are alternatives, but most other solutions will end up costing more in electricity costs. Especially problematic in say Europe right now.
RPi 3B+ has sometimes problem with stability because of higher power consumption.
Official power supply of RPi is 2,6A if I remember correctly, so 3,5 is not even possible?
Btw, many routers support sleep mode. It is not necessary to run router/connected RPi 24/7.
At 300mA, that's 1.5W of power.
Maximum current draw on a pi4 is 3A (~15W), but that's purely a power supply input limitation. You only get close to that with a ton of peripherals and heavy load on the Pi.