If you're going to do it, do it. That's the only way to keep it green. In the spring the rye will provide enough shade to the St Augustine that it will delay awakening. If it delays long enough, the St Aug may not recover, at all, in 2021. If you don't see the St Aug awakening by mid April, you need to reset your mower to the lowest setting and give the St Aug some light, so it will restart.

Appreciate the input, very helpful.