Plus if the front end is hashing with each keystroke, I feel like the security of the final hash is far, far, less secure to any observer/eavesdropper.

If the password is hunter2 and the front end sends a hash for h, then hu, then hun, etc., then someone observing all these hashes only has to check each hash against a single keystroke, then move on to the next hash with all but the last keystroke known. That hash table is a much smaller search space, then.