In what way does stability suffer? The game should never crash solely because you've installed too many mods, unless some of them are faulty. An FPS decrease is typically caused either by scripts or a large number of very high poly models, disappearing and invisible world textures are usually caused by textures with too high resolutions.
Combining DLCs is most likely going to have no effects on performance and is therefore probably not worth the effort, as long as you haven't reached the game's DLC number limit.
File size can be an indicator for optimization, but it's not a very good one. DLCs can have a very large file size if they aren't defragmented, but that only affects disc space, not the game's performance.
Vehicle model files (yft) can be larger than necessary if they include more than the necessary LODs, which does not have any effect on in-game performance, only on disc space. Checking the polygon counts (especially for L1 and lower) is a much better way to check for vehicle optimization.
For texture files (ytd), file size is even less of an indicator, as they are compressed, which results for example in multiple similar high resolution textures having a smaller ytd size than different looking low resolution textures, despite the latter requiring less memory. If texture loss happens, it is typically caused by having too many 4k textures (4 is typically a safe limit per car) or any textures with a higher resolution than 4k. Otherwise if no texture loss happens and the GPU's VRAM limit hasn't been reached, texture sizes shouldn't have any considerable impact on performance.