So what I did, using 1.4, was went to Breezehome with all three of my characters. Used 'pcb' in the console and made a fresh save. I backed up my skyrim folder and deactivated/uninstalled all of my mods. There was still a bunch of stuff left over. One, I found out a while ago that the Workshop does not uninstall mods when you unsubscribe, so I'm pretty sure some of it was from there. Also, if you manually update or change files in the NMM won't always uninstall them.
I used the other PC to verify what files were necessary to run the base game and deleted everything (not sure I would recommend doing this. USER BEWARE). I made back ups of my Skyrim.ini and SkyrimPrefs,ini. I used the Steam function to Verify files which updated to 1.5 and hopefully it checked to see if I deleted anything I shouldn't have. Updated SKSE. Went into the game and made fresh saves.
using the brand new saves, I have spent most of last night and today putting back most of my mods one at a time and testing them. I left out a couple that were causing CTD's. One was actually from before 1.5 but never found which one till now, bonus. I left out mods that I figured need updated such as any killmove mods. Detailed Cities might have been one of the MODs that was causing a CTD when fast traveling to Whiterun. If I restarted the game, then I could then get there. Have just added it back and not tested enough to see if still doing it now the Data folder is cleaned out.
Went back to using RCRN for lighting and actually it is running faster on 1.5. In fact, I have noticed overall performance is better with 1.5. Some flickering with snow in the mountains at distance.
Going to try actually playing today and see how it goes. I think I will be using my files from my last 1.4 saves because they will have data for Warzones and Dynamic Merchants.

