A couple of suggestions -
If you're using pink salt (prague #1, insta cure #1 etc) 1 level tsp per 5 lbs of meat should be plenty of cure for dry curing a flat. You do need salt (sugar helps too) to carry the nitrite into the meat, if you didn't have much salt that could be your problem.
As for smoking/getting it tender, Smoke it for color more than smoking it for temp. With that said, I generally smoke mine at cooler temps (180 - 225F) until the color is where I want it, It will prolly be on the rare side.
Then foil it and steam it in the foil for several hours (like 4+) or use a pressure cooker at 15 PSI for about 45-60 min (still foiled).
I've never tried it, but I imagine you could also use a large covered turkey roaster with water in the bottom (use a rack inside to keep the foiled meat off the bottom of the pan) and cook it in the oven if your steamer isn't large enough to hold a whole flat...
If you cook it for longer in the smoker (higher IT) it seems to me that the final product is a bit dryer
Allow the meat to cool in the foil before opening or cutting, this will retain the moisture and collagen in the meat and foil so that it will hold together once cool and still be moist.
Another thing to try is buying a corned beef brisket and experimenting with that, once you have that down, then go back to curing your own. If you buy a corned brisket, I prefer getting the leanest point you can, way better flavor than a flat!
Hybrid Pellet Pooper "The Meat Vault" - 3 RED Weber Kettles O, H (with Cajun Bandit Mod) & pre '79 - Black OT Platinum (DH) - SS AusPit - Rib-O-Lator - SJ UDS (DI) - CRAZY FAST Yeller Thermapen
We shall throw our bacon wrapped all beef justice upon the wooden skewers of truth to be held on high and let them know that we will not stand for this. ~Butz