Being a Platinum at Marriott will change your numbers quite a bit.

For example, if you use the Marriott Visa and are Platinum, you will end up getting 18 points/$ spent.

So on your stays, you would earn 14,400 points.

On your credit card, you would earn about 4800 points. That's $4000- the $800 on hotel stays that was already accounted for above, leaving $3200 x avg. of 1.5 points/$, earning 4800 points.

Since you are platinum, you could also get 2000 extra points a month as your arrival gifts.

That's a total of 21,200 points. A mid tier marriott reward stay (cat 3) costs 15,000 points (25,000 is for a cat 5). So the earned free nights per month would be 1.4 nights a month under this scenario, vs. the .64 nights a month you calculated. That would also be way better than Hilton's .93 nights a month and Starwood's .64 nights a month (where you already are platinum).

Having status and using that brand's credit card changes the entire equation.