Can you parfry hash browns in advance?
Par-cooking potatoes is an extremely common technique.

However, the shredded or grated kind of hash browns would be very difficult to par-cook with wet methods, as they would go to mush quite quickly if you tried to simmer them, and then you would have the trouble of squeezing out the water afterwards (which is necessary so they crisp up, and so they finish cooking rapidly).

Par-frying would be labor intensive, and not terribly scalable either.

Typically, shredded hash browns only take about 15 minutes to cook, which is not terribly long.

I have had some success with making shredded hash browns in a very hot (450 F, 230 C) oven, baking them for about 30 minutes.  They are not quite as good as fried, but they are still good, and the technique would scale quite well.

If you are willing to experiment, I believe that par-baking them for about 20-30 minutes at a more moderate temperature (say about 350 F, 180 C) would be a promising approach.  They would stay dry, and you could then finish-fry them just to crisp up and get some buttery flavor.  You would need try it a few times to work out the be combination of baking and frying times.