If I were to do this, I'd be inclined to take cues from double cooked fries --
- Cook them (parboil, in the oven, or even frying) at a lower temperature until they're cooked through (so a knife or skewer inserted comes out easily), but not browned.
- Let them cool, bag them up, and refrigerate them.
- On the day that you're going to be serving them -- preheat your pans in the oven as hot as it will go. While this is happening, toss the veggies in a little bit of oil.
- Spread the veggies on the pre-heated trays, and then put them into the oven. If it has convection, turn the fan on.
- After a few minutes (maybe 10 minutes?) pull out one tray and do your best to flip the veggies over. Then swap it with the other tray (so if it was on top, it's now on the bottom), and flip over the veggies on the second tray, and put it back in the oven (on the empty shelf)
- Repeat the last bullet until they're sufficiently browned to your liking.
As for your specific question -- I'm not sure of the "best" way. Boiling (or simmering) is a pretty traditional way of doing this. It can end up washing away a bit of the starches, but it also tends to make the surface just a little sticky from geletanized starch, which can be a good thing -- as you can then toss them around to rough them up, giving just a little more surface area for crisping in that final roasting.
But it's also easier to overcook them to where they're falling apart if you cook them in water. You can reduce this problem slightly by boiling slabs, and then cutting them into sticks after they've cooled down, but you'll also reduce that extra crispiness that you get from the starchy coating.
You can also par-bake in slabs, so it's easier to turn things over as they're cooking, and then cut them up. Par-baking has the advantage that it's easier to judge how it fits on the trays you're going to use, so you know how much you need to prepare.
I'd also consider par-cooking each type of vegetable individually, as it's possible that one might be cooked through before the other ones. And if you do the beets last, it'll be less mess to deal with. (I might even bag them separately, so they don't color everything else)
As for the doneness to take them -- cooked through (so no one gets an undercooked bite), but before it starts changing color significantly