Hi Susan. I'd start my planning for Slovenia by getting the Rick Steves Croatia/Slovenia book unless you already have it. (The old book should suffice for basic planning.) I love Slovenia but not sure what is of interest to you. Ljubljana is not a big city, and there is not a lot to "see" there - no real world-class museums or anything. But it's a charming, picturesque little city I love returning to. It's a great place to come back to at night.
Many people see Lake Bled. More see the natural areas around Lake Bohinj, Trglav National Park, one of cave complexes an hour or two from Ljubljana, etc. If you drive to Bled from Ljubljana, I recommend detouring through the little town of Skofja Loka (worth a stop), then driving the scenic route to Bled through the mountains over the twisty mountain roads through Dražgoše, Jamnik, and Kropa - breathtaking scenery down below! (Roads are narrow but if you take your time it's perfectly safe.)
You could also see Slovenia's tiny little coastline. The old Venetian town of Piran is delightful. There are also towns in east like Ptuj and Maribor that are worth a look if you have time.
Driving in Slovenia is easy, but if you rent a car in Slovenia and drop it in Vienna, you'll likely pay a steep one-way drop fee. One option might be starting and ending the trip in Austria so you would return the car in the same country. (There's more to Austria than Vienna obviously.) But if you just want to see Austria, maybe fly from Tampa to Ljubljana to start, then drop your Slovenian car locally and fly to Vienna or take the train from Slovenia (If you can get a direct flight between Ljubljana and Vienna, that might be easiest, but see how long the train would take.)
You could certainly fly round-trip to London and from London fly on to Ljubljana. It might make more sense to fly from Tampa to Ljubljana (or nearby Venice) and fly home from London on an "open jaw" ticket, then fly one-way from Vienna to London. That saves you the connection between London and Ljubljana which may be unnecessary - not sure. There's a good Slovenian shuttle service called GoOpti that can get you from Venice to Ljubljana in less than three hours. There are also some buses that take you to Ljubljana directly from Venice Marco Polo Airport. Flying to Zagreb, Croatia might work out better - you can take the train from there to Ljubljana.