And if you're not in a rush, you can use the monthly credit to get $5 off the Steam credit.
And then Steam has had a few discounts recently from 10-20% off the Steamdecks so you can wait for one of those (I got 10% off the cheap model back in March or so I think, and then I want to say about a...
If I'm understanding you, you can't apply the $5 reward to a pre-order, BUT when you pick up the pre-order you can apply the $5 reward and it will roll $5 onto a trade card for you.