Regarding the location of Dany’s counter attack, remember the big speech Jon Snow made when she asked him his advice. He basically said if she uses dragons to melt castles and burn cities, she’s do different than anyone else and will not create a better world. That’s why she went to destroy the supply train and the Lannister army.
The opening of the route for the Unsullied was a nice by-product, but I don’t think Daenerys thinks that far forward. If she were a better strategist, she would have saved the supply train for her army.
And now, the other by-product of her battle field decision is forty to one hundred thousand hungry, horny and stupid Dothraki. (How stupid do you have to be to think you fight better than an army that got carbonized by a giant freakin’ dragon? The only break in the Lannister lines was caused by Drogon’s fire, so we have no idea if their armored pikemen and archers could have resisted the mounted Dothraki. Note: According to this military history resource, it was only the introduction of stirrups and heavy cavalry that tilted the advantage back to horsemen.)
Daenerys’ armies are now the only major forces left in the South (aside from Dorne, a theoretical ally), so she is going to have to start dealing with military administration of the conquered territories while the war continues (an area where she has proven to be horrible). The Dothraki have been let loose upon the continent, and I doubt they have the discipline to not rape and pillage everything in their path.
There could be a crazy twist in the story where Dany has to use her dragons to protect the people of Westeros from the Dothraki.
Meanwhile, Daenerys’ next big goal has to saving Yara, and destroying Euron’s fleet.
P.S.: I thought the cave paintings showed the COTF and First Men fighting White Walkers. Who talked about them fighting the Valyrians?