Well, I think in order to trade a villager out they have to be in boxes already. In addition to that, them pinging you to move is relatively random. If there's no rumors that Tia is going to move out, there's only a very small chance that she's going to ask you to move. Sometimes even when there are rumors they're going to move, villagers still won't ping you or bring it up in conversation. There's things you can do to try and get her to ping you, like the diving trick.
Either way, if she's not already set to move, then there's nothing you can really do. At least not to my knowledge?
You could try to befriend Tia by sending her letters with gifts. It seems that villagers who have been in town longest or have a good friendship with the Mayor, want to leave the most.
In my experience, they never want to move when you want them to move.
I usually do the same thing I always do, and it's kind of by chance.
I didn't ignore Merengue, but I also didn't try very hard to interact with her.
Filled her requests when she asked, talked to her a few times a day if I see her, etc.
And she pinged to move after only like 5-6 days in my town.