The short answer is there’s a cost advantage to buying direct closer to your UY, but keep in mind:
to also consider how important UY may be to you over the years before trying to ‘time’ a direct purchase
incentives change (waiting 4 months might improve the dues advantage though risks lower incentive cycle)
date of sale together with your UY determines how many years of points are in the contract (+/- one year difference)
planning a special trip could mean buying asap is best.
Have you thought about Use Year and what works best for your travel patterns? Start here.
We like visiting different months, so the way we chose our UY was looking at which block of months we’re least likely to visit. That was summer. Sept or Oct UY looked best because most/all of our trips would happen before those banking deadlines.
That’s one part of the decision. The next part is timing your best UY choice(s) with best months to buy for limiting prorated dues. For example, last year I bought a Sept UY direct - in June. Now I could’ve pushed the purchase out 2 more months to August and saved the cost on 2 months of dues (in my case was only around $200, but higher point purchases and higher dues resorts can quickly increase that amount). That would not have changed total amount of points we got, in both cases we would’ve gotten 2022’s points and beyond; the only thing that would’ve changed was paying 2 months less dues and having to wait 2 more months before using those points.