re: Complete lawn re-do. Any way to firm up the ground before re-sodding?

I’ve always removed the old grass.
Not sure what kind of soil you got, but stick with good top soil from a reputable landscaper or materials yard.
If you want to compact your topsoil, you can rent a jumping jack, compactor from Home Depot tool rental.Its a good idea if you want to put sand on top of your topsoil for drainage.
Hope this helps.