Unfortunately you have to get in quick on these FT releases now when they come up for pre-order. Thomas will likely get reissued at some point if you can wait it out or perhaps there will be a few more for sale on release in the next month or so.
I don't even try to get the earlier releases like the Insecticons, I mainly focus on the characters that I don't think Takara will do like Skyfire, Dinobots, Combiners etc. Fortunately I've gotten all of mine when released or reissued so haven't had to pay a crazy premium. The only one I missed that did want was the X version of Sludge, that thing is in the stratosphere. I do think Takara will do Galvatron and Cyclonus but I got FT's newer versions because they just looked too good to pass up!