If I planned on using the money in December, I likely would have started pulling it out last year, or not later than Jan., in sort of a reverse DCA. Given that you are now only about 3 months out, I'd probably pull it now. Sure, you could lose a couple months of growth, but it sounds like you can live without that. If the market tanks 30% or more in a week though, you could be in a bad position. Far more risk than reward, so take it out now.
But it's not entirely clear for me that you actually do need all the money now, since you mention build time. Surely you have some estimate of total time. If that's less than about 2 years, I'd take it all now. Longer than that and I might that most now (maybe 65-85%) and then do a monthly withdraw for the next ~6 months, if that is more comfortable. I'd still likely opt for all now, but if that's a mental hurdle for you, you can placate yourself with something like that.
Also, depending on that timeline, you might look into a CD ladder, or just a longer term CD with some of the money. Rates till suck, but if the timeline is long enough and you feel comfortable committing to not needing some of that money for a long enough time, it might do slightly better than a money market or savings account.