Transfer using stand aid from wheelchair to car has difficulties with my car door threshold. Any suggestions?

Don't know the correct procedure for this or what a stand aid is. But this works well for me. Stand up turn around with your butt against the open car door. Sit down on the seat sideways. Swing the right leg in then the left and position self in seat. There is a devise called a leg lift which is basically a length of heavy duty strapping with a loop on the end. Place it over the food and use it to left either leg. Very useful after hip surgery.
There is also a gadget that fits into the door closing loop that will assist. Have not tried that yet but it helps if there is a pull handle in the roof then I can lift myself into position. There are also swivle disks that you put on the seat then the person can sit on that and push themselves into position. Don't know how they work

Teepa Snow has released a new video on youtube, "Care Partner Support Tips: Getting In and Out of the Car", perhaps it will be helpful.

Try using a slide board. There are more discussions of how to use one here:

https://www.agingcare.com/search.aspx?searchterm=slide board

"Honest John" runs a weekly Q&A column in the Daily Telegraph on Saturdays, you could try writing to him. Or, if you have a local car dealership that's part of the Motability scheme they might be able to advise on more user-friendly models. Meanwhile, if you need to get somewhere in an emergency, many cab firms have some vehicles that are suitable for people with disabilities - don't know if you can get a whole wheelchair in, but again they might have ways round the problem. I used to use main force to manoeuvre my mother into position - it was like a little dance routine - but then she didn't weight 15 stone!