Well I can answer one of your questions. Are you good with a soldering iron? You can fix your instrument cluster pretty easily. They don't sell new ones anymore (you could get a rebuilt one from gm for $500) and unfortunately the mileage is stored in the cluster. Most people will send them to someone through eBay to have them repaired costing $150-200 and a few days turnaround. You can buy a kit online to rebuild it including lights and new stepper motors, which will eventually also fail if you haven't replaced them yet.
In my case, like you the mileage sometimes didn't show. It progressed to the point where it would never power up. I took it apart and took the board out into the truck and plugged it in. Using something nonconductive (I used an eraser) I started lightly touching different parts on the board itself. There were 2 little resistors, as soon as I touched them the whole thing powered up. I added a few drops of solder and it has been good as new since.
Here's a video showing what I am talking about. In my case it was the same exact 2 resistors in this video, and others in the comments said the same thing. You can save a lot of money doing this one yourself..