Does the soundbar not have a sleep timer? Seems counterproductive to sell the product boasting one remote controls it all when it won’t let you do a basic sleep timer function.
If your device does not offer a sleep timer option, add one yourself. Buy either a electronic or mechanical plug in one rom your favorite hardware store.
I had a television that came without this feature at one time. Rather trashing it, I just bought a Mechanical plug in one. I used to set it to turn on just before I expected to arrive home! It worked out just fine.
I concur, the roku already has a power function and a timer. Should be straight forward to combine the two.