At 165 lbs and 5'9", you definitely have a lot of room to grow naturally, but if you're set on the Deca route, 300mg is a solid dose for a first run with it. However, 500mg of Test might be overkill and lead to more water retention and aromatization than you want.
A more "modern" approach to avoid the dreaded "Deca Dick" and keep the gains lean would be to keep the Test lower, maybe around 200-250mg (just above TRT), and let the Deca do the heavy lifting. Also, since you mentioned being a hard gainer, don't ignore your digestion. Deca can sometimes slow down GI motility for some guys.
Adding 25-50mg of Proviron as @2Thick mentioned is a game changer here—it keeps the libido up and helps with the "dry" look. Focus on the calories, brother. Deca works miracles, but only if you're eating in a 500+ surplus.