[Galaxy Tab S Q&A, Help & Troubleshooting] Merancapeman:
So, let me begin this tale with this disclaimer: I've worked with modding communities across a whole span of genres, from gaming to building computers. I will begin with the assumption that I am at fault in this case and I'm clearly doing something wrong. I've about reached the end of possibilities that I can explore on my own, so it's not like I tried one thing and gave up.

Once upon a time, I knew more about what I was doing than I do now. I first bought the T800 model several years ago, and the first thing I did was learn how to root it and flash CWM recovery. I went on all sorts of adventures with the device but never fully explored the potential; just installed awesome stuff that I thought I would use forever and got rid of bloatware and other useless crap. Flashforward to a few months ago, when I realized I had a really nice tablet that I had spent money on but didn't use like I had wanted to. I'm working more and more on audio mixing, and I wanted to install an app for the tablet that would allow me to remotely operate a mixing board. To my dismay, the app required an updated version of Android, and I hadn't a clue as to how to do that because the tablet was rooted and such and the thing wasn't going to update (was stuck on stock 4.4.2). I don't remember what I had done with it many years ago - at least not very well - and all I had learned was nearly gone. I refreshed a little, made a backup, checked to see what I had and noticed I still had the CWM recovery up, things were still rooted fine, and I was prime for flashing a new ROM and wiping everything. Found the Lineage ROM, slapped that puppy down, aaaand... Verification wasn't happening. Huh? It didn't even try to install, it just gave up and put the stock ROM back on. Tinkered with it for a second and thought, eh, whatever, I'll just let it update itself now that it's been wiped and everything. Only the device still wouldn't let me update it to Marshmallow or whatever update limit it's got.... Awesome... Did a little more research. Oh, my recovery is probably super out of date. Grabbed ODIN, slapped a brand spankin' new TWRP recovery that was geared for my device on it, and decided that I shouldn't go with Lineage. I'll just go with a stock Nougat ROM (can't post a link, but if it's important I can PM it to you if it's relevant).

Followed instructions to the T (as far as I'm aware) and this time did everything with a fresh new outlook into what I was doing. Flashed it, flashed GAPPS, aaaaaand.... Samsung logo won't go away.... -sigh-... Wiped it again, tried to make sure everything was wiped correctly, made sure everything was up to date. Tried again, but alas... Nothing working right for me.

Perhaps during this tale you've already picked up on something I royally messed up. Perhaps you're thinking, "Wow, how did this doofus not brick his tablet?" Am I getting old and stupid? Was there still research I missed? I started with the low road of easy modding and had to force myself to a higher road of deeper learning and STILL came up empty-handed, so now I'm asking this awesome community for a hand. I know you can't do this for me, but I'd appreciate a guiding light, or at the very least a link to an article that I really should have picked up on. Let me know if there's information I've missed that I should also be putting out, and thanks for reading regardless!