Here's an idea for the weights of sideloaded fonts. I don't know if it will work since I haven't tried it, but the folks who use sideloaded fonts may want to test it out.
There is a file called .kobo/Kobo/Kobo eReader.conf. I noticed the following line:
Maybe, just maybe, you can try changing Amasis to the name of your sideloaded font and it will change the weight for you.