I reread your post several times because I really wanted to understand what happened to avoid a similar situation. We can't use banked points for different resorts in the same booking even if they are all in the same UY? I'm newish so I haven't dealt much with banked points yet. But I have smallish contracts (all under 120 points) but all same uy and when I make my bookings it just had me list the order of how I want to pull points from each contract. And it does seem to automatically use bank points first before pulling from current year points. Obviously I'm making my bookings within 7 month window so all can be combined, but now if I understand you correct I would run into issues with banked points at different resorts?We have two UY, Sept and Dec. We have two resorts in our Sept and one in our Dec (which is the same as one of our Sept). I recently had banked points for both of my Sept resorts and also for my Dec resort. We had five reservations that we cancelled just because we are going to save a bit of cash and not go. I didn't want to lose any of the banked points so I wound booking two nights at one resort and two single nights at another resort since I couldn't combine the banked points from two resorts in a single reservation. I sent an email to see if they could link the two single night reservations and they did it. Today decided to check the reservations and found they can canceled the one reservation with banked points and just made another night with the points from the other resort. So I now had orphan points that I wouldn't be using and homey don't play that game. So I cancelled the newly made reservation and made two single night reservations like I had originally. Luckily, the nights were still available. And I sent an email asking "Continuing Reservation" be added to the two one night reservations.
One UY can be easier, but more than one resort in a single UY can still cause problems when you aren't expecting it.
Going back to OP's original question, I purposely kept all my contracts same UY for the ease of using all my points interchangeably when within 7 month window. I can tell you I struggle a little already booking 8-11 months with a small contract for resort advantage. Like most people have said already, having multiple UY would work great if you use your contracts to make separate bookings. Also buy a large enough contract to complete your trip, otherwise you can bank and borrow or I guess transfer points too. My account looks a bit like a complicated mess since I'm usually pulling points from multiple contracts to book, but so far haven't run into any trouble.