Is there a reason you aren't looking at air travel? Really like the scenery, really don't like to fly?
Those bus rides are long enough that flying could be economical.
I didn't look at train options, but for your long runs they could also be worth considering. You get the scenery, but you can get up and stroll better.
I also use Rome2Rio a lot, but have usually found when I'm actually at a place that there are more bus options possible. And sometimes some of the R2R options aren't available because of track outages or strikes or such.
Have you tried guide books? Sometimes some more possibilities will show up in them. Free in your local library, or well-priced on Amazon. You need some anyway for the other stuff. I personally like the Lonely Planet series for what I call "trip logistics", but there are lots of other excellent series.
Or just searching on "Salta, Argentina to Buenos Aires, Argentina bus"?
Do watch out for Weekend, Sunday and Holiday/Event travel. Lots of schedule variation there. Usually just less runs, but sometimes very few. And for Holidays/Event they buses can be pretty full.
And then there is the country specific areas on this site. You might browse there for now, but if you don't get enough useful responses here you could ask the moderator of this area to move your post to the South America area.