Slime is heavy. If you have really good kevlar tires please try a decent set of normal bike shop
tubes. Not Walmart, not super hvy duty either. They make the tire get way hotter at speed on the road and they are heavy. You only need hvy tubes for trail riding thorns or to make up for cheap tires. I run servas drifter S tires with standard tubes. They have 5 layers of flat protection and inverse tread. Use good rim strips. I have zero flat problems. You want to keep the spinning weight at a bare minimum if you want to go fast. I have no balancing weights and standard weight spokes on good rims and hubs. If you have good quality wheels they are too light and well made to need balancing. At over 20mph about 30% of your HP is being used spinning the spokes thru the air. Fat spokes equals slower speed. Look at a Olympics Bike, that's why they have 3 wide thin spokes. If your not jumping rocks good quality spokes in the standard size are fine if
you have a shift kit or one of Jim's Hub Adapters so your not torquing the spokes themselves. I use a $25 3/32 drive chain with riveted pins for the shift kit. Standard shimano hub/cluster. If you look at a Walmart bike rear hub closely you will see the bearing on the cluster side is set nearly in the middle of the hub. Shimano hubs run the bearing close to the outside. It's spreads out the support on the axel. Whether you like shimano or not they are a much stronger hub for a reasonable price. Dishing the wheel for a cluster has nothing to do with it as long as the spokes are tight and right. My bike will run with mods including shift kit,CNS carb,tuned pipe,slant head,opti-2 oil/75 to 1, at 42mph in a calm wind on a dead flat 4 lane road. That's clocked by 2 different cars not by a bike speedo. I ride a steel GT triple triangle frame just like yours. Broke a costco bike frame last year. I have no suspension at either end not because I wouldn't like it. Adding a suspension fork to a bike that didn't come with one, esp if it's a long travel fork is dangerous. Bikes with no suspension have less fork angle than a frame built for a suspension front end. As the suspension colapses the angle gets less and less. I promise it's the slime unless your new fork is a suspension fork and the frame isn't made for one. It sounds like you have done everything else right. It will cost you less than 20 bucks to prove I'm right or wrong. I weigh 220lbs. I have over 2500 miles on my bike including the rebuild.
I'm not trying to ruffle any feathers. I'm just saying what works for me. I do Zero offroad.