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Another point to consider is: don’t over think it.
The belt will move around bit and that is normal. If you’ve got the bike on the ground, the tension is good, the axle is even, that is all you need to worry about. When all these factors come together, the belt naturally rides within the confines of the pulley.
Guys tend to get lost in the trees. They’ve never paid attention to what the wear marks on the pulley look like, or how the belt rides during normal operation. Now that you’ve done the tire install you’re hyper fixated on something you’ve never paid attention to before. So step back, know the belt moves around a bit depending on load, tension, direction, temp, etc. This is normal. Your job is to ensure the axle is straight, and the belt has the correct tension. Then the belt will fall into place.