you have a couple options, one is more technical than the other.
The easiest option is to use Geoff's patch and enable advanced font control -
https://www.mobileread.com/forums/sho...d.php?t=260100 that will let you have access to the advanced font control for Bookerly which you otherwise wouldn't since it's a sideloaded font. You can increase the weight of the font which thickens it.
The 'harder' option is to use an application like font forge to adjust the font. I don't know the particulars of how to do this.
I'm sure others will argue the merits of each method.