Gary Fountain

You do sound cramped on this frame although a photo or two of you on the bike would be nice to see. It would be far better for someone experienced in bike fit to actually see you on the bike.

Seatposts can and do snap and extending the seatpost beyond it's limit is placing undue stress on the seatpost. If it's an alloy post and it does snap you don't get any real warning which can see the rider dropping down onto the broken stub of the seatpost. OUCH is an understatement! If it's an 'aero' post it is just that little bit more dangerous due to a finer cross section. You sound like you're pretty tall and are probably heavier than other riders due to your height along with the natural ability to put more power into your pedal stroke which puts you in a more dangerous position. Pushing the pedals forward results in pushing the post backward (and sideways) which is a primary reason for posts to snap due to metal fatigue. If you're lucky you fall off the back of the bike along with the saddle and the top stub of the seatpost.

Have a look at your seatpost and look for any nicks or cuts which is a natural place for a crack to start. if you do find a flaw in the seatpost you could polish it out thus minimising a starting point for a crack. Infact - chuck the current seatpost for a longer, sturdier seatpost if possible.

I reckon you're doing the right thing by trying to swap your frame for another frame. It's easier to go bigger than smaller.