Seat setback question- would a larger frame help?

I feel like the answer should be obvious but I’m struggling so thank you for any help.
I currently have a 58cm Specialized Diverge with an aftermarket 20mm set back post and still need the seat set all the way back to get to my optimum position (per a professional bike fit) . I’m looking into buying an Aethos and noticed the seat post setback is only 12mm. I’m in between frame sizes per Specialized charts so I thought about going up a frame size but if I’m thinking about this correctly, that wouldn’t change the set back right? My seat’s height from the bottom bracket would stay the same as well as the seat tube angle and therefore I would be in the same position but just with more height made up by the stack instead of the seat tube?

I plan on testing both sizes when my bike shop has them in stock and changing the seat post to something with more set back is always an option but just wanted to check that I have this concept right.

  • Assuming the seat tube angle spec between both sizes is the same, correct… there is no change in effective or possible saddle setback options. Ignoring other changes, the only difference could be a taller top tube (but I know there is far more than that in reality).
  • Correct.

If you are really aiming to get further back, you need a slacker seat tube angle and likely a longer setback option seat post.

thompson make a range of seatposts with large setback. as long as its a round tube you should have no trouble finding the right post.

Thank you!

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