"Big difference for example, between expert Python dev and expert C++ dev writing display drivers or graphics engines."

I'd encourage you not to sell your earning potential short by negotiating against yourself like this. C++ display driver writers sound more impressive and might be in shorter supply, but so are the companies that need them. Even if it's legitimately harder!

There are a lot of companies who bring in a lot of revenue off of web-stack stuff that need Python devs, and so are in a position where it makes sense for them to pay a lot for that skill set, and that's where the value side comes from.