Safe waiting time between FCO arrival and then train to Florence

There are no return tickets in Italy. Except for some same-day offers.

Discounted tickets came with restrictions related to changes and refunds. Purchasing tickets On the day of travel, you will pay the BASE fare with Trenitalia and the Flexi fare on Italotreno.

You can't pay more, once published Base and Fare do not change. Both fares are flexible before the departure, after the departure you have a leeway of 1 hour with Trenitalia and 2 hours with Italotreno. For obvious reasons nobody buys a Base/Fare ticket in advance.

If you are seeing prices in $ and not in €, you are not on the official sites. The price of airport trains is fixed: 14 and 8 € per person, one way. So you can't pay $ 99 via Termini and $ 42 via Tiburtina if you are taking the same Rome-Florence train.

I don't understand what you mean with "Booking about 30 days out", either. I have just checked on trenitalia.com Today there are many SuperEconomy (27.9 € pp) and Economy (38.9 € pp ) tickets left for Rome-Florence trains running on September 5. Since discounted tickets are capacity controlled, you have no way to know if there will be still SuperEconomy and Economy tickets left 30 days out.

If you miss a connection between two trains run by the same company because the 1st train is late, you are always protected. Even if you have purchased a discounted, non refundable ticket.

Get the Trenitalia app and book your tickets once you’ve landed and gone thru passport control

No risk

2.5 hours is probably enough but no way to predict

I plan to use the ItaliaRail or Trainline app. to book my tickets because they are in English and seem easier to understand.
So unlike train reservations I have made on Trainline in France, Belgium and the UK, the fares are not lower when booked in advance, one to two months out as opposed to few days before?
Is there no difference in cost of fares for a reservation made for less than 7 days, 14 days, 21 days, etc. in advance?

Trenitalia is in English too.

Discounted tickets (Super Economy or Economy) are available until sold out. When they are sold out only Base (full price) fare is available. Sometimes discounted tickets are still available just days days prior sometimes they are gone long before.

Just be aware that if you buy a discounted ticket then you are stuck with that choice. If your plane arrives early you cannot travel on an earlier train. If your plane arrives late, you cannot travel on a later train. So the risk is that your flight is late and you lose your investment. If buying a discounted ticket it is best to have a large cushion of a few hours.

If you decide to buy at the time of arrival, then you almost certainly have to pay the full BASE fare. You can buy at the ticket window or the automatic kiosk at the FCO train station. You don’t have to use the app.

Both the Trenitalia and Italo websites have English language options and are easy to navigate. Anyway, if I were doing this, I wouldn't book the Rome-Florence leg until I hit the ground. There are too many variables for my comfort. (As has been noted, you don't need to book the Leonardo Express in advance. Just get your ticket at the kiosk at the airport station.) High-speed trains run regularly between Roma Termini and Florence (Santa Maria Novella), so once you land you can choose whether to book with Italo or Trenitalia. Out of curiosity, I checked tomorrow's high-speed trains and found availability around noon as follows:

Tenitalia: 50-58 euros for "Standard"
Italo: 44 euros for "Smart"

All of these tickets can be purchased at the airport station.

As others have said both Trenitalia and Italo websites and apps are also in English. I used both for the first time in March. I had the exact same question you had, trying to figure out what was a safe time to book the train after arrival at FCO. After much (probably way too much) analysis I decided to use Italo trains, booking non refundable tickets in advance using the discount code sent in weekly emails I got after setting up the account. I bought tickets online from the website but all appeared seamlessly in the app.
After misreading someone's advice, I chose to book the Italo fast train from Tiburtina, and used trenitalia to take the cheaper/slower train from FCO to Tiburtina. (8euro). I got such a great deal on the Italo train I actually bought 2 sets of tickets separated by 2 hours, knowing one of the sets would be thrown away - because the 2 sets of tickets together were less than 1/3 the cost of the full price ticket...
The bad news is, our original flight from Boston was cancelled so both sets of tickets were 'thrown away'. The good news is our flight on Aerlingus was cancelled due to mechanical failure, so Aerlingus reimbursed us for all of our additional costs, including the full price train ticket the next day (plus lunch and one night hotel that we didn't get to use) and compensated us more than what we paid for the tickets (I didn't ask for that, it's just standard, and I guess they use average cost, not actual cost in determining compensation!) AND it turns out we were able to take an earlier train the next day than what I had originally booked for the previous day because arrival, passport control (no luggage claim, carry on only) customs, and walking through the airport to the train station was a lot faster than I estimated.

In the long run, the discounted tickets were a waste, and going to Tiburtina saved us 6euro each but looking back on it, the advantage of Tiburtina was marginal and that station doesn't have as much to offer in terms of food and shopping, although it was less crowded than Termini. So, if you want to save the most, yes take trenitalia regional to Tiburtina and book Italo train (usually cheaper than trenitalia) and if you really want to save that you're willing to gamble throwing away the ticket, book ahead with discount code, but for peace of mind and not having the stress of wondering the whole time if you'll make the connection, just wait til you get to FCO to buy your tickets, but definitely use the apps, they're both easy.

FWIW Mar16, flight scheduled arrival 11am FCO, (landed early but got to gate about 11) we had time to spare taking the 12:50pm Italo train from Tiburtina to Florence SMN. BUT your mileage WILL vary. This was just lucky and I would never bet on that. My original 'earlier' tickets were for 3pm departure from Tiburtina.