If I had 800k SPG points rolling in a year, I'd burn some to get to Diamond. I'd figure out how many nights shy of 50 I was and get to a town that has a Cat 1 hotel that offers 250 points/ night for declining housekeeping and book 3 rooms using points, checking in on a Saturday and phoning in my checkout. It would cost 17.25k (19k - 1750) points to earn each 7 nights toward Platinum. Given the SPG Amex gives you 5 nights toward status, the most points this strategy would cost for a 15 night Sat-Sat stay would be 108.75k. (Beforehand I'd chat with a hotel manager to ensure there's no issue with holding a room I'm paying for but unsure how long it will be before I return.)

After achieving Platinum I'd probably burn points on a local Cat 2 because I'd need a lot fewer nights to maintain the status.