I have no way to test this with the equipment I have, but what about opnsense on an x86-64 box and throw an sfp+ pcie card in there. You could then in theory turn off auto negotiation and set it to 2.5g. Has anyone out there tried this?

I’ve been running opnsense with my CenturyLink 1g setup, though I’m still using their ont to convert to copper, and been very happy with it.

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I’m pretty sure sensi thermostats, when controlled by home assistant, do what you want. I haven’t confirmed they don’t still try to phone home, but that could be dealt with by some firewall rules. Other thermostats that support homekit should also work.

I do almost exactly the same, except I have opnsense running on a cheap dual nic mini PC so I don’t have that dependency on my proxmox servers. The unifi stuff does need a controller, but they publish a free app that you can run instead of getting their hardware.