I think it is legal to hunt dove with black powder in Texas. With dove hunting getting so commercialized and competitive I think switching to 1.5 gauge makes a lot of sense.

Larger calibers would result in fewer lost and wounded animals thus making the sport more humane.

Should I load with 7.5 shot or 4?

That could expensive if they ever ban lead shot.

I think #4 would be overkill. If you plan on finding a bird that you hit with #4 with some meat left?

Just use pea gravel.