@MissNadine It's not really weird that the game can handle what R* designed it to handle. The game engine can handle the whole map because the whole map isn't really loading in all the time, at least not the high quality stuff you see around your character, they use a lot of LOD, SLOD for the buildings and surrounding roads, grass, etc to reduce the strain on the game engine. A lot of mods being made do not use LODs or SLODs, and if the mod author used a high draw distance, the game is trying to load more data than it can handle, so you start seeing stuff not spawn in.
I think they didn't optimize the game for PC, just kind of ported over what they had already made to work on consoles to work on PC, and didn't take advantage of the possibility that most PC are more powerful than consoles. That is I think what the main issue ends up being when you try to install too many maps along with the other mods. I have to assume that the Titanic mod you have is a pretty big file, and if it is somewhat close by your maps, and it is trying to also load in even if you are not that close to it, if it has a high draw distance, it could be using up some of what the game is able to display. The game also uses occlusions, which blocks other things behind the occlusion from being rendered since you aren't really looking at objects behind other objects. That helps keep down the amount of resources that have to be used. The ymaps most likely are not in an area with occlusions or don't have occlusions, so the game is most likely trying to render everything, even if you aren't directly looking at it. I'm not sure if it's possible to make our own occlusions, I'm still in the process of learning that stuff.
Best thing to do to keep down the frustration of conflicting mods or poorly optimized mods, is to add them one at a time and test them out in the game and make sure no issues pop up. Then add one new one and retest in game. If all is well with those two, then add the third and test. Just keep doing that and if you find a mod that causes issues, remove it before adding the next one. And I'm not really sure, but I don't think a map mod can conflict with a scripts mod. But scripts mods can have their own issues if poorly coded or if one of the items that the scripts depends on isn't up to date.