If You want increase the weight of custom fonts You can patch Your Kobo.
https://www.mobileread.com/forums/sh...d.php?t=313422
Among patches there is one of my favorites: "Freedom to advanced fonts control" which let You choose the weight of all font, not only a sub set of system fonts.
If You decide to patch, of course, there are many other possibilities, for example enable rotation on all devices (not only on Forma) is very useful in my opinion, or custom font sizes ecc.
The alternative is to weight Your custom font and then download in Your Kobo, in this forum there are some tread about it (using font forge or other systems), but I am not able to do it, and patches are a more complete solution because You can choose each time the desired weight.