With 18x UHD binos, I would look to upgrade your chest glass first since that is the optic that you will use the vast majority of the time. Optics set ups are very personal decisions. If money is no object then I would pose two threshold questions to guide you. First, do you enjoy looking through a spotting scope? If not, sticking with high quality 15x, 18x, or saving up for a BTX will be your best bet. I have been on several hunts on second choice or OTC hunts where simply finding game as opposed to evaluating game is your biggest challenge. Slowing down and staying on glass with both eyes open for long periods of time with a high powered optic is unequaled for spotting game. That and a 15x and up will allow you to judge from a fairly surprising distance. The second question is, can you hand hold 12x binos. If so, save yourself the weight and go 12x and spotter. If not, you will do just fine with an 8x or 10x on the chest, a 15 or 18x in the pack. If you have designs of carrying all three, be diligent about the weight of the optics you choose and think about a 8x32 or 10x32 on the chest.