If you wanted an actual true damage combo with up throw at 0 its this one

Up throw > drag down up air > regrab > downthrow > fair/bair.

If you know that the opponent isn’t mashing airdodge or jump you can do this combo

Up throw > dragdown upair > regrab > downthrow > nair > regrab > downthrow > fair/bair ( or alternatively up throw > upair if you want a juggle situation)

you can also change the nair into a first hit of fair into regrab depending on % and weight.

Up air > up B is good when you know that they aren’t the type of player to mash out of hitstun. Otherwise they get to land for free cuz there is decent endlag on the up B.