So i agree that you started a little high with the frame but what a lot of guys on this forum are unwilling to try that i have tried and know people to have tried is buy all parts from aliexpress.  Most people on this forum stop on the frame.... they think that that is the only corner worth cutting because of the stigma of fox and rock shox.  I do think that such a build is still possible but you will have to do a ton of research.  There is a company on aliexpress that sells shocks called Himalo and having worked in a bike shop for many years and taken apart many shocks in my day cannot tell you a single difference between Himalo and other name brand shocks.  It wouldnt surprise me if they are the same thing.  I have ran them for a little over six months and they are incredible. for 180 USD  you can buy himalo shocks at 100 to 200 mil travel and they are incredible. that would save you a ton of money.

No, that's not right. A steep effective seat tube angle does not necessitate a shorter effective top tube. The gold standard trail bike is the Ibis Ripmo, eSTA 77 degrees,  eTT 632mm for a large. It's literally the most awarded trail bike out currently.

A shorter eTT is a design choice and there's nothing wrong with that.