My load order is large and extensive, and yet I am able to runs settlement that are about 10x larger than diamond city without issues other than frame rate. I know why I don't crash all the time lol, gaming rig and choice of mods are two very important reasons.
I have older mods in there, and what order are you referring to? That order has been working for me for the last year or more with zero conflicts, zero game errors, and no crashes despite the occasional ones that always happen even with unmodded games. I ran my load order through loot, can you guess how many red conflicts?
Three. That's with 2500 hours playing the game. That isn't the loot approved load order though, since it wouldn't let me apply it due to an error I couldn't understand. That's the original load order, in all its messy glory, and it's been perfect.
One of the red ones being a warning that you mentioned about the new snap and build esm that I don't have downloaded. Pretty sure I'm doing it semi-right if the only issues I'm dealing with are after downloading sim settlements and rotc. That's why I'm here, to help fix them.
Apologies if I came off sarcastic or in a non optimistic way. Not the case. I'm just really surprised your game functions at all. I've spent a fair amount of time troubleshooting my own load order over the years with various bethesda games and have come to a realization that even the smallest errors can cause massive issues as games go on. I'm currently sitting at 330 active plugins, 251 of which are either esm and esps, and the rest esl. Other than some read error saves that I can't get fallrim tools to open (which isn't causing crashing amusingly), I am 95% crash free. If a settlement crashes for me, it's most likely due to me running into a heavily modded cell that I'm not giving enough time to load. Usually rare, and a reload almost always fixes it.
To point out a few issues with your load order (and I don't use loot myself as the variations in load orders can get too complex and random, I think it's crucial to build your own list and check it yourself in f04edit).
I know for a fact that this esp right here basically overrides every other weapon leveled list for any boss characters for any other weapons above it (also several vendor lists).
*mjp_PTRS41ATR.esp
You probably have a bunch of boss level characters running around BOOOOOMMMMing everything in a two kilo radius with that rifle. BOS, Gunners, and the occasional raider who was supposed to be carrying a missile launcher. If you don't, it means the leveled lists are being overwritten by another mod as I don't see any merged patches in your load order. I personally converted that esp to an esl and stripped the leveled lists out as well as most of the vendors. Great gun, but the leveled lists are insane, and any other weapon mod you might have that edits the same lists is being overridden (if it's not script injected, but still issues with that mod regardless).
So you've been playing your game for 2500 hours without actually getting the proper experience as many of the mods below any mods that override them omit the data they carry. You'll have a much better game experience if those conflicts are resolved. Might not fix your SS issues, but it will make your game more stable.
Not going to look through your entire list with a fine tooth comb, but I can quickly point out a few problem mods I've removed from my own list due to serious errors.
*Gatling Laser Overhaul.esp - Mesh issues or something, causes random crashes. Fun but buggy. There was one mod in particular that was a mess...I forget as it's been so long.
*WelcometoGoodneighbor.esp (used this once a longgg time ago and noticed it doesn't play nice with a lot of mods) - Even the SS nexus page has this: Welcome to Goodneighbor - This mod has a number of problems. With Sim Settlements it can cause the Call Town Meeting function to get permanently stuck in a black screen. It can also cause your settlers to quit functioning.
*Survivalist GoBag.esp - Eventually will cause your armor bench to crash when scrolling over a bag. Repeatable bug. Shame as it was a favorite. I'm back to the old backpack mod, but it works great.