Well when I play any of my characters solo, I just use the same bindings and UI layout. The only thing I have to do is focus that character on itself and all the macros work just as they would in a group setting. Unfortunately that doesn't really help with a respec and different playstyle.

In that situation I would set up additional action bar sets using Trinity Bars and just load whichever template I wanted to use. I haven't tested it, but I believe keybindings to the action bars will save with each template, so that should handle the switching.

Now I'm not completely certain about this next part but... The UI used to be limited to 120 buttons, or 10 bars of 12 buttons the way the default UI arranges them. A recent revision to the UI system got rid of this limitation I thought. This should allow you to set up one template with the first 120 buttons (or however many you need), then set up a second group that are for action button 121-240. What this would be important for is that spell/macro action button locations are stored server side.

The only thing I see as a hassle if when you respec and lose talent abilities, they'll be unmapped from action buttons. These will have to be set up each time. I suppose one way around that issue is to make a macro for all talent spells. Those won't be force removed from your bars.

And just a warning, pretty much all of that above is unconfirmed. Sorry if any is wrong or doesn't make sense, but it's coming up on after-lunch-I-really-want-to-take-a-nap time...