Thailand forum

I'd say there are definitely some great things to see in Krabi (Railey, Ton Sai) and the Phi Phi islands, but it is true that getting from the East Islands to the West will require a day's travel, so cutting your time a bit shorter on the East (specifically Koh Phangan) would be necessary to make it worth it.
My advice, especially for those traveling solo, is to leave itineraries loose; everyone wants to see as much as they can in the amount of time they have, but there's no way of knowing what will happen when you get going, and itineraries can often be a source of stress. Especially when you're alone, you will often meet other travelers that you get along with, and it's better to be flexible and travel a bit with others than sticking to a plan you made having never been to the country.
Have an idea of what you want to see, and go from there! You will have a great time regardless.

As I recall, there is no overnight bus, primarily because the portion of the trip that takes place over land actually only takes about 4 hours or so. The kicker is the ferry rides, both of which take about 2 hours, give or take (Koh Pha Ngan to Surat Thani, Krabi to Koh Phi Phi), and don't take place during the night, for obvious reasons.
I'd forgotten about the 5 day booking thing around FMP, though the case may be that a lone traveler might be able to book a room upon arrival for a few days, though I'm sure you'll pay a premium for it regardless.
Maybe if you get your fill of island after 10 days or so, you might do well to head up towards BKK and see some of the surrounding things like that previous post suggested, such as Kanchanaburi, which is super chill and beautiful, or Ayuthaya, which is very historic and interesting. There is, after all, more to Thailand than the parties and the beaches! (though both are a blast for sure)