The key here is Miami needs to be smart, Miami needs to make decisions with every inch of calculation made when it comes to the QB position. The Miami coaching is not going to make any decisions just because (I don't think there will be any JUST BECAUSE here).
People think of Fitz as a 3rd rate untalented hack, but he's not, when he's on, he can compete against anyone...If anything he's been inconsistent in years past, but (HUGE BUT) lately Fitpatrick has come on, and is looking more like a positive to have on the team, then a negative.
My thoughts are, the plan is to start the season with Fitz, allow him to work with Tua, to help him grow week to week, help him see what to expect in the NFL, and also to use that vet experience to help him through the 1st few games with an O-line that will probably have every player playing for the 1st time with each other.
There is no real hurry, though there is huge expectations to compete and win this year. What I mean by that is, there is no real hurry to get Tua in there, or should there be...Miami is in a great situation, why not use it. Fitz has been playing well with little talent, now allow him to start the season with with MUCH better talent then last year. Expectations are still very high, so if Fitzmagic turns to Fitztragic, Tua will be waiting in the wings, but if he plays well...You ride him as far as you can.
I personally believe the plan is to start Fitz, and if all goes well, and Tua proves himself, bring him in after the 1st quarter to half of the season to replace Fitz. This is what I think they may want to do, but as I said, if Fitz gets hot, and start winning, or at least playing too well to be removed, he will not be replaced until he falters, but on the other hand if in practice, and preseason (if there is any games), Tua comes out of the gates worlds better then the other 3, it will be very tough for Coach Flores not to start Tua from week one.
This season's main objective is not to get Tua on the starting team as soon as possible or to sit Tua, and keep him there to learn as long as possible, but to be as competive as possible, and if everything falls into place win a SB (Like 31 other teams), and Coach Flores is not going to make decisions on Tua to please the fans that want him to start or to sit, but make the best decisions for the Team he coaches in 2020.