A few things FWIW:
I'm 5'9" and ride a size 54 2018 Synapse Carbon Disc Ultegra SE. I got the size S Topstone 105 (came with Ultegra, as described by others). This means that I have to raise the seatpost quite a bit on the Topstone, which means in turn that I end up with a way more aggressive position on the Topstone than on the Synapse. Having said that, I absolutely adore the Topstone! I've been doing long rides on pavement, including some group rides where the others have carbon 4 times the price, and been struggling on the climbs (I admit) but killed them on the descents. And I've done plenty quite rough fire roads, single track (up to blue mountain bike trails) as well as small bike packing adventures. Totally love the comfort, and how it practically runs on rails on descents. Having a 1x12 mountainbike, I have come to the realization that I prefer 2x setups, which was the reason for getting the 105 version.
BUT - on technical descents, I really wish I had a dropper post.
So, a question that hopefully some techie/insider has the answer to: if I wanted to retrofit a dropper post elegantly on the Topstone 105 size S, what should I look for? This model seems to have the specs of the one on Apex bike: http://www.tranzx.com/product/10dropperpost/ysp20j.html
Can anyone confirm? Are there openings to route it internally despite the 2x system on the 105? Any restrictions on how far the seatpost can be extended on the 365 mm version?
If that's not the exact same post i'd be very surprised. All of the specs share the same frame set and have routing for the internal dropper.
Not sure on minimum insert ect, perhaps an owner on here can advise.