I'm probably alone on this(?) but I do the grain filling first, before staining and sealing. That's certainly how the hardwood floor finishing guys do it.
It's been awhile but I think they use a stain & filler combined product.
https://www.mohawk-finishing.com/products/wood-touch-up-repair/fillers/grain-filler-solvent/
It's been awhile but I think they use a stain & filler combined product.
https://www.mohawk-finishing.com/products/wood-touch-up-repair/fillers/grain-filler-solvent/