You're getting great advice on how to make good tasting bacon. I've also made some tough bacon.
I don't think your process is making it tough.
Belly, like butt, is not a tender cut. Look at the recipes to make braised pork belly. They use time and moisture to break down the collagen in the meat and slowly render some fat and develop browning.
Commercial bacon might also be mechanically tenderized in addition to small cure injection needles which have the same effect. Cutting against the grain is another form of mechanical tenderization mentioned by others. I find it difficult to find the direction of the grain sometimes after smoking. A trick I've seen brisket cooks use is to make a notch before rubbing and smoking to indicate the direction.
As meat ages it naturally tenderizes. That is one of the benefits of dry aged steak, but also hams and other cuts of pork.
What does all of this mean? You might have just gotten some tough pieces of belly, but there are a few additional things you can do to get a more tender result.
1. Age the belly. As others have said, take your time at every step.
2. Use a jaccard to get some mechanical tenderization. It might help the cure penetrate too.
3. Cold smoke the bacon to get the smoky flavor without losing moisture.
4. Sous vide the whole hunk of smoked bacon. This is used when cooking uncured belly to get a tender, yet moist result. Something like 36 hours at 145 should give nice results. Cool it in an ice bath when done. Then refrigerate and partially freeze before trying to slice. I've made seriouseats.com sous vide pork belly porchetta and it is life changing. Sliced thin and cooked like bacon, it makes an amazing sandwich. It shows how tender yet crispy belly and bacon can be. I've got an unsliced chunk of bacon I'm going to try this on today now that I think about it.
If you don't have sous vide, you can improvise and try cooking your bacon in a pan or sheet tray with just enough water to cover it. Once the liquid cooks off, the browning and crisping happens just like normal. I've found this helps when I've noticed a batch was tough. The more water and longer it takes the better.
Good luck!
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