Nice work. Especially referencing the TI prior art of the Speak and Spell. This kind of synthesis was quite prevalent in the early 80s - school BBC Micros had a ROM which let you "*SAY" a phrase. Classic Macs had MacinTalk.

Another codec which might be interesting to try but is considerably more complicated is AMR, from GSM: https://en.wikipedia.org/wiki/Adaptive_Multi-Rate_audio_code...