how long would you cook this in the slow cooker? would like to eat on new year's eve, around 5:30, and need the chance to make gravy from the pan...

I'm interested in knowing how this turned out . . . .

It seems like a difficult recipe to convert because the recipe starts with a very hot oven, after which there will be a period, once the temperature is reduced, that the oven will retain the heat from the high setting. You simply can't replicate the cooking process of the recipe by searing at high heat and then putting the roast directly into the very low heat of the slow cooker.

I might add here a note of caution, on a similar note. I was quite disappointed by this when roasted in my oven, which has seemingly perfect insulating properties - so good that it must have taken many hours for the temperature to drop to optimum slow cooking temperature. Not only was the roast overcooked, but the glaze became quite bitter. Just a warning . . . . ;o)

Otoh, recipe already calls for 6-8 he cooking overnight at low heat (after initial few minutes at high heat).
Sounds like slow cooker timing.

Have a look at this chart for converting (regular) recipes to slow cooker or crockpot. Then fit into your schedule.
https://www.onegoodthingbyjillee.com/crockpot-conversion-chart