Are the Twins that much of a huge improvement over Riverrun?

House Frey was always the stepchild house and House Tully was much more prestigious. Strategically and geographically, the Twins are better but going by prestige and power Riverrun would be better.

At any rate, I think Tyrion and Jaime played Bronn.

Tyrion knows if Cersei wins, he’s dead and his offer doesn’t mean anything. Also, Bronn doesn’t get anything at best, sentenced to death at worse.

If Cersei loses, Dany or whoever becomes King/Queen has to approve giving away the castle to a mercenary that was fighting for the Lannister’s at the Battle of Goldroad, and who shot Drogon, and who didn’t join the Battle of Winterfell. Tyrion has merely a suggestion. If Bronn gets the castle, sure, everyone wins. If Bronn doesn’t get it, he can’t do anything as killing a King/Queen advisor is stupid and a death sentence and he loses that crappy castle he already has.

Offering Highgarden would ensure Bronn jumps on his deal and takes extra risk. It also gets a reaction from Jaime which helps sell the false validity of the deal.