There are lots of mini splits designed to be placed on the floor, for the reasons you've described - having short walls and angled ceilings. They get installed right above the baseboards generally. A quick search for a floor-mounted ductless mini-split will give you some results.

Twintech Heating & Cooling

https://twintechheating.ca/

A mini split system is great option for your bonus room. Mitsubishi offers a floor mounted indoor unit specifically for your application. You can read more about them here: https://www.mitsubishicomfort.com/products/indoor-units/floor-mounted