Hello, what is the "best difficulty" ever reached by any device on Bitcoin?
Best regards,
Ssatooshi
You are going to have to find that in your node's data. I don't think anyone is going to run that.

also, reached vs recorded are two different things, recorded will be on the blockchain, reached may have happened outside of the blockchain, like an orphan block, most importantly, none of that matters, exact diff or a fraction higher earns you the same block reward as having a 10000x higher diff share so none of that matters.
With all things being equal, the higher your hashrate, the higher the diff shares you get. 609M being your highest means your miner is small (which you did mention anyway), I just checked mine and it's 7T ( a single M30 on solo mode), while 7T is a freaking large share, sadly, it wasn't enough for a block.

With that said, your power rate is tempting, 2c/kWh is pretty low; however, I assume this is limited for home use? i.e, you can't host 50 or 100 miners, I'd assume.