CTs would be the easiest option for longer term monitoring. Installation is non-invasive (to the wiring), and you can either buy a power monitoring system or design one yourself.
For something portable and non-permanent, a current clamp can be used to monitor a single device.
For a quick check, you could use a kill-a-watt
All the small little quirks that aren’t well documented or documented at all. Then I’ve got to go back through my notes and lessons learned, but invariably I’ve forgotten to write something down and have to resolve a problem I know I’ve solved but don’t remember.
New hardware can have quality or design issues as well. Demonizing used hardware is a bit extra
If WiFi worked before, there’s no reason it won’t work with a new ISP. It may be that the router provided by the ISP is crappier than your prior one, just buy your own router or AP