I don't ride my bike upside down so it's never a worry.
I know what you are saying, but a few seconds right side up and a few pulls on the brake lever and all is well again. My mountain bike is sitting in my garage upside down at the moment. It's been that way for 2 weeks now as I'm waiting for some new wheel parts to come in. When I turn it over it I know for sure the brakes will be spongy. But once I pull the lever a few times all will be back to normal.
Storing hydraulic disc brake bikes upside down is not an issue unless there is already an existing issue with the brakes (poor quality or damage).
Bottom line here is that all good bikes are going to be disc brakes if they aren't already. The rim brakes will be left on the bikes you buy at Walmart.
Here's just one example. Almost the whole line is disc brakes save for couple cheaper versions of the Domane
https://www.trekbikes.com/us/en_US/b...-bikes/c/B200/