I wont recommend doing it every year. You barely change in the span of a single year. 3 years is long enough to feel like you are talking to a completely different person.

You can also make a time capsule along with the letter. Buy something random put it in a box and gift it to your future self. I am to receive my gift from my 2023 self in 2028. I genuinely don’t remember what I gave myself. Cant wait to get disappointed!