Optimal interest rate environment for mortgage-backed securities?

Ideally, both those risks are supposed to be priced into the market rates already.

So the optimal time to invest is a time when the market expects interest rate volatility in the future but you know better.

If you don't have a crystal ball, it's hard to tell. There's certainly no general answer, since even with (hypothetical) perfect knowledge of future risks it depends on which risk profile you need/want, given your goals and the rest of your portfolio.