Yes, the limit is to avoid longer loading times. There are only 250 vehicles loaded at once and each click on the "load more" does load 1000 more vehicles.
=> The button does have good reasons.
If you have to press the button (almost) every mission – even on small ones, you may want to overthink your system and try to spread your vehicles a little different to avoid that issue. (You of course don't have to do that and I also understand how the button can be annoying)
Ah thank you, its a shame the limit is so low. For the most part, my set up is as realistic as I can get it, so spreading them out unfortunately isn't really an option. If it's client side and not MC side, Then definetly an option they need to add.