I don't think any of the Roush or other aftermarket modified cars will retain the value or be as appealing as the GT350. Those cars sell here like mad, I'm in the car biz. I enjoyed my 2015 GT PP that I had prior to my SS. Things I hated: rear wheel hop, rear brake squeak every 10k miles with new Ford pads, terrible shifter even with a MGW (non-issue with proper tranny in the 350), and the exhaust sound (another non 350 issue). I had it 1 year 9 months and 28k miles. One test drive in a 17 1SS 6M and I was sold. I do miss the more useable trunk and more useable interior of the Mustang.
My best friend in Savannah had a GT350 for a while and was/is a HPDE driving instructor in the southeast. He said the car was fairly boring on track compared to his Boss. They have been seeing engine issues with that flat plane crank engine, but being able to rev a V8 like that has to be a blast.
I think having two ZL1's would be kind of whacky myself. I do really like the new 18 Mustang GT performance pack 2. It starts at $44.999 just like the 1SS 1LE.
Since you sound like you like to flip cars, this might be out of your consideration, but what about a Civic Type R?
GT350 out my way
https://www.autotrader.com/cars-for-...odelCode1=MUST