I downloaded a couple of gorgeous custom lots that would be a hassle to build from scratch. For example, I downloaded THIS lot (or/and something similar from the same creator). I unselected everything besides the lot itself on Clean Installer because I already have the majority of the CC and even if I don't, I have something similar to prevent cluttering my already large CC collection. The lot would load in a normal speed for my game, it wouldn't lag, it will have those replacement objects for the missing CC which I don't mind. As I delete them the game simply crashes. I tried it again, same happens. I try redownloading the lot, same issue. I remember it also happening with a lot I can't seem to find it and I deleted it recently so I can't even find the name. It had a similar aesthetic like the previous lot only more leaning to mid century. It was very realistic and it had an underground garage. I removed all the CC an it still took forever to load only to crash when I wanted to clear out the replacement objects and edit things like flooring and walls so the family gets a fresh start. I was so sad that it was unplayable. What causes this kind of crashing? These lots are CC free or containing popular CC I have no problems with. What can I do to prevent it in the future? I have downloaded plenty of larger and more cluttered lots from creators like SunnySimblr and didn't have problems with crashing, plus I would do the same decluttering method with her lots.

Would be careful with removing groundcovers, as those can cause issues if removed (usually flashing blue ground). Otherwise it should be mostly safe to remove CC (could be a few other similar issues, can't think of others at the moment).

I assume you have Apartment Life and M&G (or the UC) installed? If you don't, lots made with these or any higher EPs than you currently have, will crash your game even if installed via Clean Installer.

Most lots I've had problems with that were CC-related or related to the lot either crashed while loading the lot or immediately on entering the lot, not after a while of editing (unless they were large and caused RAM crash, or had CC issues). Are you entirely sure the crashes are related to the lots, and not other issues like those that would otherwise cause memory issues or other such problems? Do you get those kinds of crashing problems when doing other things than editing lots?

So should I keep terrain paints? Not a problem.

I have both of them, not UC but the version where the packs were downloaded individually with 4GB patch and Graphics Rules Maker. I also have the Super duper hug bug fix.

Like I said, it only happens to certain downloaded lots and it's consistent with them. I had memory problems and now I have 3GB free, which is still low but I'm working on it. I recently downloaded a huge farm wedding venue by Sunny and didn't have any issues besides the lagging which I fixed by removing lots of decorations. That lot also had CC but never cause an issue. I'll try downloading the crashing lots again unfurnished WITH CC and then remove the lot's CC folder once the lot is loaded and saved.

You could benefit from having a test game, especially when editing lots.

One trick you can do with the lots is to have a game folder only for installing lots (rename the "The Sims 2 (UC)" folder you currently have by adding a word at the end, then run the game - the game should regenerate an extra, new game folder).

In this new game folder, you can install lots with CC, remove CC manually and test to see if they work otherwise, and repack for use in the other folder.
You can also test the repacked lots from Clean Installer.

When you're done, rename this folder with a word at the end ("TEST" or some such), then remove the extra word you added for the other folder, and you'll be able to use your regular game again. (it's important to rename in that order - always keep one or both folder renamed at all times, or one could try to overwrite the other, or do other weird things). The folder that has the original name is the one the game reads.

This way you don't have to do anything with your current downloads (they read the same install files, but otherwise the folders are fully separate).

You may want to copy over mods like the hugbug mod and other fixes that directly influences house building and the like.

Very occasionally missing CC will cause a lot to crash. Not just terrain paint but other objects can too. Most times unticking is fine, but its something to be aware of.
I tend to install iffy/don't know type lots into my test folder as Simmer22 detailed above. I then use the globe cleaner on it to make sure its free of bugs before repacking it for my main game. This is moistly cc filled lots or lots from creators I don't know.

That sounds like a great idea except that I don't have enough memory for that. When I free up my memory I'll try the TEST thing. I guess for know the only thing I can afford is to install with all CC, sell the unwanted objects in game, save it, possible in a bin, remove the original build and delete the CC folder.

Crashing only happened when I would not turn off the game for days but rather just minimize it and play it whenever I'm free so I don't have to wait 45 mins to load. This hadn't happened for months. Crashing also happened when I would go through hairs and accessories cause I had to many but ever since I deleted 30% of both it doesn't happen anymore. So basically, crashing right now only happens with those lots.

Memory/RAM won't be influenced.
Harddrive space will, though.
Those two are different.

A test game without any CC (or with minimal CC, as in what you'd need for each custom lot + essential mods) takes up about 1-3 GB of harddrive space (depending on setup and how much CC you put in).

A test game with minimal CC would load much faster than your full game with all the rest of your CC, so the testing and cleaning of the lots would be easier and much faster (my current full game takes 40-50 minutes to load, while one of my low-CC test games takes about 3-5 minutes depending on how fast I click stuff).

If your hard drive is so full as to not be able to do that you may find that alone gives you issues. i would make it a priority to clean up and make more hard drive space.