"I know I could just go ahead and replace the thermostat but I want to take a break because I've already done so many other things lately..."

What follows is just one opinion...

While 90C is a bit high, and 78C seems a bit low, there's no immediate danger, and your engine will keep running while you take a well-deserved break.

Watch the gage to see if it goes even lower when outside temps dip further.

Find/borrow an IR temp sensor to follow up on other posts, and to double-check your gage's calibration.

Best Regards,
Jim