Because the devs never added any bass clef games for sight reading or sight singing, and I am a bass, I went looking for an alternative.

I found this website: https://sightreadingmastery.com
that allows you to choose what clef you want to sight sing in (either bass or treble) and they have musically valid exercises, as they hired composers to create never-before-seen melodies for each grade level. So not only are the musically valid, they are also of a consistent hardness within the hardness level (yay!)

It is less fun (not as gamified no scoring system) but for someone who really wants to learn, it is fun enough. If @ToneGym Community does come up with a game I will use that one as I would then renew my tonegym pro membership

At the very least the devs could allow a bass clef option under training preferences for Solfègiator

There is a trial at sight reading mastery that allows free training for seven days.
Or for those of you who like to spend money, a paid option that IMHO is rather expensive, but also comes with a lot of other benefits, including one-on-one training with a professional once a month.