I will start with 500 sandpaper? Then work myself downward?
Absolutely not! Sandspaper is numbered like so. the higher the number, the finer the grit. So 500 is a very fine grit used in the automotive paint and refinishing trades, rarely for wood as the wood particles will clog it instantly.
Having done this recently, this is a major task, not a typical DYI project for someone who has never done refinishing. JMO. That doesn't mean with some technical help and physical help, to remove the door and seal the opening either with visqueen or a temporary plywood door, you can't do it. Sanding off the old finish by hand will take for ever and unless you can lay the door flat and see what you are doing and have gravity working for you, it will be very difficult.
There is a product called "Refinisher", it disolves the original finish and can then be blended and refinished. It's made by Klean Strip and it's available at Home Depot, $5.49 per quart.
You can try on a small portion as see how it works. As was mentioned the reason for removing the door it to allow the stripper to sit on the flat surfaces to react. If it runs down it won't work as well.
These types of oak door are beautiful, but they require considerable upkeep to keep them in good condition. To let them go is to destroy them...not good.