I bought a Pixel 5 two years ago for exactly that reason (fingerprint on the back) and its small size. I love this phone but honestly the fingerprint reader works 70-80% of the time depending on sweaty hands etc. and by now I wish it was on the front because it often lays on the table and I have to pick it up. It all has its pros and cons and I think an ultrasonic under the display is the best option (right after apples face id).
Nice! Is it planned to release it on f-droid or any f-droid repository? Appreciate the work, still using Infinity for Reddit! Edit: Found a comment from the dev over on reddit that he plans to put a version on f-droid! Awesome guy!