What’s the ‘buying old server hardware’ version? Ideally something I could pull stats from and record monthly usage.

The cheapest way to get your sum-total consumption with granularity in the range of perhaps 5 minutes would be to use an RTL-SDR (around $15) to listen to the data transmitted by many utility meters.

Next step up to collect per-circuit usage might be to purchase something like Emporia Vue.

I think the best approach is to go with standard “linked” detectors with hardwired signal+power, and then for automation you can get a “bridge” to will forward the signal from the wire to your home automation system.