This could actually be a good solution, one version would be standard and the other optional?
It sounds very reasonable, assuming there is space for a button on the back that does not compromise structural integrity. But since your panels are so thick (which I really like), and quite a bit of space above the motherboard IO, then it might not be a problem.
It probably also comes down to how convenient it is for you to offer different SKU:s. Your main manufacturer of the case (the metal) probably does not care if there are different panels, as long as the differences are small, comparable cost wise, and can be handled by the same tooling process. I guess they just want a minimum amount of money, in order to go ahead?
For individual parts to complement the case, you probably need to meet other MOQ:s. In case of a power button, it will be part of the case regardless of option, so your quantity is deterministic.
However, let's say you also offer a front panel with full IO (power button + ports). The IO module quantity would not equal the case order quantity, and become non-deterministic, which could be a problem?