The bossnut evo is available in a 15.5 inch small as well as the medium might be worth a try.
Edit.didnt read that you thought the medium was too short.ive a medium santa cruz and the medium evo feels similar if not a bit longer.my mates 5’10 and loves his medium bossnut.ask your local go outdoors if you can try it with a seat post with a bit of layback off another one of there bikes most of the bikes they sell are 30.9 seat post so something will fit this may make it feel more roomy.