I have such monitor too and have no problems with such big DPI scale set in the system. In fact it affects only fonts on desktop and maybe in internet browser allowing to read text from the distance.
But yes, if you want to stay with some low value, then font in windows mentioned by you will remain small. However I would expect those fonts to be as small as other fonts in the system (by your choice)
You can also check development builds which have HighDPI support improved. Maybe it changes something.
Anyway, here is how Reaper looks on my end (click for full size):