Downstairs sink gets scalding hot. Bathroom sink (which is 2 ft from shower) gets scalding hot. Shower gets medium hot at best. Any non-plumber fixes here?
It sounds like the shower has a thermostatic mixing valve. These do have an adjustment to change the temperature limit if you want hotter water. I must say however that showering with very hot water may feel great, but is not really good for the skin.
Research the matter, but I would say that the temperature of full hot from a shower should not be over 110 F, but evidently the code is 120 F maximum. https://www.phcppros.com/articles/1828-hot-water-system-temperatures-and-the-code
Hit the cold water shutoff just before the offending shower, then set the water as hot as you can. You should get much hotter water. That means that you either have a thermostatic/anti-scald mixing valve or the fixture itself limits how hot you can set the water. Either way, it should be adjustable or you can swap the fixture out (although swapping fixtures may fall into plumber territory).