I've only booked twice with Marriott.

Once at a Courtyard in China. Once at JW in Singapore.

Both were with points. Both were wide open for booking with points, for my dates and around those dates. Easy peasy to book.

I think you shouldn't jump to conclusions just because the hotels you want aren't available for the dates you want. SPG hotels probably also release only a certain percentage of rooms for C&P.

To investigate this further, try searching up to a year out for one particular hotel, and then try again for another, if you haven't already done so. If you already have and can't find any, then it's probably hotel-specific.