TBH I don't know what to expect here, from the SZCT drawings it would look like the seat tube is quite short compared to my other bikes so I was hoping I could fit at least 175mm, but I might be proven wrong. I guess my legs are quite regular.
Looking at the technical drawing from SZCT it would seem that the seat tube starts to bend backwards at sth like 22.5cm of insert on M-sized frame
CEF50 seattube length is the same as other Size M modern bikes @410mm (Transition Relay, Trek Fuel Exe) generally come standard with 150mm dropper.
I would suggest on your current bikes, measure the length from centre crank spindle to centre seatpost rail.
From there, you can calculate and get a better idea on what you need. Seattube length + dropper post from collar to rail.