Yeah, Sim Settlements is one of those things that should be officially incorporated into the game. I started a new playthrough, purely to test it out. I wished I'd had it the first time I played through the game. It seemed to impact my machine's performance when I was in "Starlight City."
If you want to do a roleplaying project with Fallout 4, I recommend looking at Alternate Start or Another Life, and Pip-Pad. The first 2 let you begin a game in a variety of locations, and with different starting gear. I didn't use either of them enough to know which one's better. The 3rd one replaces the PIP-Boy with a handheld tablet (mostly cosmetic, it functions the same except it removes the light, so you'll need a flashlight mod).
Some "QoL" mods I liked:
Towbie's Realistic Weapon Sounds
Lowered Weapons
True Storms
Darker Nights - a cool interplay between True Storms and Darker Nights was fighting enemies in a thunder storm, waiting for them to be illuminated by lightning
More Where That Came From Diamond City Radio
Fallout 4 Enhanced Color Correction
A Touch of Green
Better Settlers
Diamond City Enhanced
A Better Third Rail
All of the above are self-explanatory, I think. I would set up any new Fallout 4 player with these mods.
Basement Living - Allows you to "craft" a variety of subterranean shelters in Settlements. The shelters are "plug and play", and come equipped with beds, workbenches, and other stuff. I liked them because there's no way to get in out of rad storms with regular Settlement buildings (True Storms allows you to make radstorms really bad news, which I did). These underground shelters are instanced, so Companions will come with you, but Settlers can't get in (so they can't steal your bed, for example).
Some Assembly Required - This one makes powered armor rarer but tougher. You find pieces of power armor where you used to find whole suits, so you won't get a whole garage filled with suits of armor. Assembling a full suit of X-01 is nigh-impossible, takes forever, and I suppose it must turn you into a leviathan (I never did it, so I can only guess how good it must be). Fusion Cores are still way too common, though. There's a mod that allows you to increase (or decrease) the consumption of cores, but I'd recommend a mod that slashes the number of cores that you find instead.
The Beantown Interiors Project - This lets you access many, many of the abandoned buildings that are just boarded up in the vanilla game. Most are filled with junk, so much junk you'll never run out of junk.
Subway Runner - This makes the Commonwealth's metro/subway system accessible and fills it with monsters and loot. It should have been part of the vanilla game at launch.
There are also a couple of mods I used that added a variety of knick-knacks to Settlement building. They didn't improve the broken, maddening mechanics at all, but they added variety. There's also a mod that allows quick-saving while in Survival Mode, but I can't remember what it's called. There's also a good flashlight mod, that lets you dial it up to a friggin' blinding spotlight or down to something reminiscent of the flashlight in Half-Life 2 (I went with the latter, naturally). Again, I don't remember the name of that one.