I just did a little test with my recent "new game" (supereasy, cheaty free-build^^):
Downloaded some settlement Mods that include:
the Overlook, Drumlins Diner, Concorde Parking and all settlements extended (which may be relevant here, there are tons of other mods as well).
I started with the overlook (that can be seen from everywhere), did a manual "Fortress in the rocks" with two high towers and and overall of 29 Settlers.
Went down to Sanctuary, where Codsworth managed the settlement to Lvl3 and I additionally placed advanced heavy industry (5 plots + IDEKs Logistics) on the water, left of the bridge (heading to RR) and an additional Living Block (4 floors with 5 residental and 3 recreational interior plots). 27 People living here.
From here I went to Concord Parking, which I kept compact. One high block with 18 residentals (int), some industry in the back, a row of commercials, some recreationals and a lot of turrets. IDEKs Logistics heading to the road to concord.
I continued to Abernathy, it currently is level 2, so I don't yet have the locations where to start my expansion.
I went on to Drumlin Diner, where I built industry in the back, commercials on the top and a large 3 (split-) floor centre on the road supporting 34 settlers with a lot of additional stuff like individual bars, shops and recreational spots.
Still stable as a rock.
OK. Now for RR. I made Preston build it up. The first thing after 0 was ready, was to put IDEKs Logistic station directly opposite to the one in Santuary. I are just about five plots between the two. On that left side of the road I installed a greenhouse with 6 Agri interior plots, 4 Industry plots and some small martials. In the middle I built a small living-tower for 14 aditional settlers, built a junky fence around it to adjust to look to fit the rest of the settlement and spawned me 20 settlers.
I left for the south and played quite some time at nordhagen where i managed to build a really beautiful seafront with 24 different stone and brick homes (some self-made with interior plots) some industry in the back and a really nice shopping mall with recreatinal plots. Supporting overall 24 settlers.
One crash related to a missing (weapon) mesh here when designing the shop clutter
Let the "city" grow to 18 settlers and went back up to Oberland and let it grow to lvl 2.
No more crash. Went back to the triangle, lots of shuttering (I have Fo4 on a slow disk) until everything was loaded. Down to ~15fps average and some seconds of beeing stuck. But after about three Minutes close to Drumlin, back to 60fps.
RR made it up to three. I can see all surrounding settlements from the additional "tower" I set up on top of the lvl3 top-cabin.
No crash, no real problems, but:
-Abernathy farm didn't advance even 1% further, all needs are filled, everything is available, and - same as RR it doubled the desk and some foundations/floors. I let it bebuild and have a look back there in the evening
- IDEKs Logistic stations don't fill with clutter, they work as intended, but look empty.
- After three weeks in game, I didn't notice one info about industrial revolution (all materials are more than available^^).
So I was confident and resaved one of my perma-ctd games, loaded it and guess what: It took just a few seconds to crash, loaded my first, unmodded game (I got stuck in the mass fusion building after the epilogue). This also immediately led to a crash right before all mods where loaded.
Final thoughts (for now) on the topic:
On some machines the triangle may be too much to handle, but the CTDs have to have other causes. Otherwise my extended triangle (or circle) couldn't work that fine.
I guess there are problems with plot-advancements, when plots advance to a level where meshes or textures are missing, where npc do scripted things, where the scripts are missing, etc.