Like others have said, I suspect a current Cat 6 hotel certificate will not become a new Cat 6 after Aug 1, but what that 7nt certificates becomes is anyone's guess until Marriott comes out and says. I would think if you're lucky it becomes a Cat 1-5 certificate and if unlucky a Cat 1-4 certificate in the new system, and then you'd need to pay more points to upgrade (if they still allow that). The current Cat 6 30K/nt points doesn't translate well into their new categories.
If you are looking for 5 nights at Swolphin, that will be 48K SPG per room (5th night free) if you book before 8/1, leaving you with 28K SPG (3x = 84K Marriott points). And then you can use 60K Marriott points to transfer for 25,000 Southwest RRs if you want. Just ball-parking and guessing $200/nt at Swolphin, I think that gets you $2000 in value in hotel rooms + ~$320 Southwest = $2320 value, and 24K leftover Marriott points. But if you can pay less with URs then that alters the math quite a bit, though URs are so much more flexible.
Alternatively, 124K SPG = 372K Marriott. If you get a Cat 1-5 travel package, you'd get 120K Southwest RR (which I estimate to be worth ~ $1540 in airfare; I take the points and divide by 78 to get a value for them), a 7nt 1-5 certificate with an unknown value at this point, and 102K Marriott points leftover. Those points would be good for TWO nights at Swolphin after 8/1, a value of ~$400? So, a total of $1940 in value plus whatever the 7nt hotel certificate is worth. If it's worth $380+ then it's at least break-even with the above scenario, if not then you're better off booking Swolphin before 8/1.
If you used 372K Marriott points strictly for transfers to Southwest RR, you'd get 150K RRs worth $1,923 and 12K leftover Marriott points.
I guess part of the question is, if you like split stays would you want to spend 7 nts at Swolphin? Plus you'd still have to pay for another room, right?