First of all, Welcome Aboard!
As for buying with the "X" Plan, the amount off is really not that much. I purchased my 2025 ST in January 2025 with a MSRP of $58,350. At the time I was a member of the Mustang Club of America which offered the "X" Plan as one of it's many bennys. The ST Dealer took the "X" plan amount off the top of the vehicle's MSRP, line #1. I'm sorry to report that was only $2,100 OFF the sales price. I reminded them that this was my 2nd "X" Plan purchase (2016 Mustang GT Premium, $45,000 minus the then standard of 10% off for a savings of $4,500 I received). The ST Dealer then gave me their "New Customer Discount" which gave me $2,500 more. Clearly, I was disappointed thinking I was getting 10% off like previously. The ST Dealer commented that the new ST's were hard to get at that time and were selling for approx $5,000 to $7,000 ABOVE MSRP here in Nor Cal.
If this purchase becomes a "no-go" issue for you, why not look for a Ford lease return that is a Certified Pre-Owned vehicle? You get a good, well maintained vehicle at a much lower price that has a 7yr/100k mile power train warranty in addition to what ever is remaining on the original MFGR warranty. Just saying. . . . . .