Hey there! Apologies for getting back to you so late. I've been awfully busy.
Based on the information given, it sounds as if the crash is wildly inconsistent, but the mission itself is clearable. Does the crash seem to happen when you reach a certain stage, or a certain area? If it seems to keep occurring when a certain part of the map is reached in a certain stage, it's quite possibleyou might be hitting the billboard limit. This can also occur in the vanilla game, and has a high chance of occurring in Level 5 near the Monorail area if the player gets a Hit & Run while on a mission. Unfortunately, I can't seem to find any video footage of the crash in action at the moment, but I've seen several speedrunners report it.
What cars and collectible items are being loaded in the mission? Does this crash seem to always occur in the stage where these cars or items are activated?
If you can, feel free to send the mod over privately into my Discord DMs through a file sharing service such as Dropbox or MEGA.
Thank you!