The seat tube length has no effect on the fit. The three dimensions to consider are the TT length, the seat tube angle and the head tube length (with the headset). A steeper STA increases the frame reach, for a given TT length.

You have the same inseam to height ratio that I do, but your inseam is 4-5cm longer.

When you mention that the saddle is pushed back all the way, that tells us nothing. How far back is the saddle tip, relative to the center of the BB? At your saddle height, a setback of 7-8cm would be relatively far back. If you have a nonsetback post, the setback might not be that extreme. It may also be too far back. I assume that it's set to place the knee over the pedal?

I'm not sure what you mean by a 45 degree stem. There are several angles that can be measured, but even a +17 degree road stem would only produce about a 34 degree angle relative to the ground and that would be be a very high rise.

I'd say that your fitness level is a problem. A racer with a saddle height like yours (about 78cm?) might have a total head tube stack height of 190mm and use a 73 (-17) degree (horizontal) stem with no rise to produce a 12cm drop to the bars.

Even though I'm 9cm shorter, I have my saddle tip 6-7cm behind the BB and I still use a 110mm stem with a 53cm TT and 74.5 degree STA. My stem has not rise at all (73 degree).

A rider of your height should not need a 53cm TT and you will not find any stock frame with a TT that short that has adequate head tube length.