Just tried on XSX, doesn't seem to work unfortunately, maybe due to the new character select screen for next gen? So it would need to be done with a 2nd character that has the final mission ready.

"Witt has responded on discord regarding this, they're planning on adding the category soon." -a message from user Whizzo on another thread: https://www.speedrun.com/gtao/thread/kauty/1#0sgyd

Other vehicles are allowed for the preps, but entering your garage to get a different vehicle on the finale was banned from the beginning under the assumption it would be patched in the following update, meaning those records would become unbeatable.

Not sure the best place to put this, so hopefully this is okay.

The kosatka prep mission can sometimes be the one most likely to go wrong, due to the higher risk of dying. The longfin prep on the other hand can pretty much be skipped by using a phantom wedge and job warps to teleport the trailer straight to the drop off. Removing the police can also be done as soon as you collect the trailer by accepting an apartment heist invite, making this mission very fast and easy.

On the finale you can choose the main docks and drive round to the drainage tunnel and enter there as normal. The only limitation I can think of is then not having the scuba suit, making swimming slower.

Perhaps this has already been tested and I'm just not aware. Sorry if that is the case.

Before starting the prep, you need to open the map, hover over the invalid job and press to start it. Wait for it to load and it will bring up an error message which you can accept, returning you to freemode. Then when you start the prep, the job icons will still be there.

I would like to add that it can be done on console too, though the actual method for warping is slightly different and unfortunately slower compared to PC.