If you are no longer plan to travel, what value do MR points, as a currency, have for you? Why would you be considering getting the Gold card? Why not, as others have recommended, just cash out via Schwab and shift to cash back going forward. Seems getting a Gold, with a SUB, just muddies the waters because 1) you don't plan to travel and 2) you will not be holding the Schwab card and thus no longer have that as a cash out option.

Regards

P.S. Also, it should be noted that you actually cannot downgrade a Schwab Platinum to an Amex Gold, not possible. So that actually isn't an option. You can do that from a vanilla Platinum but not a co-branded one.