Hey
@John P -
Welcome to the site, and welcome to the FrSky side of things.
As stated above, FrSky does not limit their radios based on the different models. They all have the same core features, all have the same number of channels and all have the same programming. I know coming from Futaba that makes little sense, but FrSky looks at the products a little differently.
FrSky radios run on a firmware called Ethos, it is very powerful, but also very friendly. Give it a test run, and pretty quickly I think you will find it much easier to use than your Futaba. There will be a little time to adjust to the new system, but think you will be pleasantly surprised with how enjoyable and simple it is, even when doing really complex programming.
As you mentioned FrSky has a VERY detailed manual you can download. It is outstanding, and really handy should you get stuck on something. But in general I find most people get along with the radios just fine without it. The author of the manual does tend to frequent this site from time to time.

. (He used to write manual for us in the OpenTX days.)
So when picking a model of transmitter it comes down to the added features you may want. Honestly the X14/S radios are great. I like them a fair amount. The X18/S radios are the next step and they main attraction here is the touch screen, and that is really nice to have. (Didn't think I would like it that much, but having a little touch keyboard to name your mixes and models etc sure is handy.) Then the X20R/RS have a little bigger screen and a slightly different layout. The X20 Pro AW is super deluxe with a lot of CNC aluminum, adds Twin and Tandem RF options, and a bunch of other goodies. Basically the radios will get a few more switches and features and get a little bigger dimensions as you climb up the model numbers. You just need to figure out what you would like on your radio.
I'll add that the "S" models outsell the non "S" models. The selling point is usually fancier gimbals. But I'll let you in on a secret, even the standard gimbals are better than the gimbals on your old Futaba.
I personally have an X18 and LOVE it.