i didn’t know TK had 2H layouts when i installed it years ago. i think they both have a lot of overlap in being feature-rich, with main differences (imo) being:

Thumb Key

• has more 1H and 2H layouts, variations of layouts, and languages
• is a tiny bit more responsive with fewer mistypes for my thumbs, but YMMV
• can be further modified with YAML, which i haven’t tried

Flickboard

• uses long-press for 0-9, instead of a separate pad or dedicated flick gestures, which i find more intuitive
• can upload images for keyboard backgrounds

i think Thumb Key has a higher ceiling if you’re that kind of power user, but Flickboard has like 90% of the same goodness with an important extra little bit of aesthetic flair