Sounds great. What about hardware acceleration features of the NIC? I read somewhere that its better to disable the support for that in OPNsense when running it in a VM?
Obviously you would install Linux on it. Absolutely no difference to a regular server then.
Maybe stating the obvious, but don't skip over older laptops if you can get your hands on them. Especially because the CPUs are already optimised to be power efficient. They also come with a built in UPS that even in bad cases will still be good enough for 15 minutes or so, which is good enough for 99% of the power cuts. Only down-side: lack of SATA ports, but that is more or less the same with mini-PCs.
https://github.com/fatedier/frp seems to be designed for such cases, but I have not tried it myself.