I am also in this sector and will need to use FBM for new launches and as a back up for slow inventory receiving. My plan is in late Sept/early Oct to switch to FBM on one product and secure those 25 FBM orders so that I retain the FBM option through Q4.
Are you able to do this? Are you setting up with a 3PL? If you are unable to launch and get these 25 sales before 14th October then you will be limited to FBA. It would be worth considering airfreight for a small number of units to achieve those FBM sales and ensure you have both FBM and FBA in your tool kit.
This will affect launch strategy so must be viewed as part of the bigger picture. When do you anticipate having inventory available for sale at Amazon?