Well I found something. I found a page on the steam forums where a user reported his similar issues were fixed when he ran SSME - Skyrim Startup Memory Editor. I downloaded and dropped the two necessary files in the my Skyrim folder and launched Skyrim. Upon exiting to an exterior cell, it loaded for about...two seconds longer(?) before CTD. So that's progress I guess. MemoryBlocksLog, however, does finally show my Default Heap set to 768! Joy! Doesn't fix the CTDs, but I'll take what I can get. I noted that the game loaded up to 360MB before crashing for whatever reason, so at this point I can only assume that it's probably not a memory issue, or at least not entirely. More than likely it's some texture conflict or whatever that I may have caused when I first realized something was wrong and tried reinstalling lighter versions of some of the more memory-dependent mods.
So, I've decided that I'll just make a separate Skyrim install and start over, this time being a bit more mindful of what I install and how I do it. I'll keep the older install as-is in case someone has another suggestion to the culprit and might know how I can salvage my current setup.
EDIT: Forgot to mention, I also tried loading Dragonsreach on a whim, since it's one of the larger interior spaces with decent amount of clutter and people, and also got an immediate CTD upon attempting to load. So that screams "mod conflict/error" to me, particularly since I'm able to load the Abandoned Prison (the starting location in Alternate Start - Live Another Life) just fine. Probably SMIM or some such, which leads me to think I'm better off scrapping this build and starting over with a bit more discretion.
EDIT 2: In the process of reinstalling SKSE, as well as other mods, I think I might've found out what was causing my initial memory issues. As other posters above said, this issue is usually caused by SKSE being improperly installed, which I think was the issue in my case as well. I downloaded the pre-made .ini file of the nexus, (the one referenced in the STEP 2.10.0 guide, and while installing the archive I noticed that the STEP guide seems to suggest that the file path should be \mods\skse_1_07_03\SKSE\skse.ini, while the filepath that comes with the premade .ini is \mods\skse_1_07_03\Data\SKSE\skse.ini. My guess is that since I downloaded this pre-made .ini off the skyrim nexus, it was packaged like a standard mod, with the file contents stored within a Data folder so it could be installed through MO. My guess is this discrepancy in file paths kept SKSE from locating the custom .ini file, so it just ran on its default settings, leading to my CTDs. There was a link to an image of the appropriate file structure in the STEP 2.10.0 skse install guide that probably would've tipped me off to this on the first time around, but the link is dead. Kek.
Modding sure is an adventure.
FINAL EDIT: Final update for anyone interested. I tested my earlier theory regarding the skse.ini file structure through MemoryBlocksLog and it seems to have confirmed what I thought: The addition of the \Data\ folder in my SKSE installation was what was causing the .ini settings to not manifest in-game. Removing the \Data\ header resulted in the settings functioning as intended (I edited the Heap memory allocation just to verify, and the MemoryBlocksLog.log reflected those changes). So yeah. Don't install things like a dingus. EZ.
Edited by Cbode1, 01 June 2020 - 11:16 PM.