Tkumpathenurple
Veteran
If you're going to take apart the PS4 Slim, you're probably much better off taking the PSU and the AC converter out, and use DC converters instead.
Otherwise you'll be wasting some 15% of your battery pack converting from DC to AC and then another 15% at the PSU from AC to DC.
According to ifixit's teardown, the PSU only has two DC voltage outputs, 4.8V and 12V. There's a chance that your power pack has one of these voltages as configurable output (or even both, which would save you a ton of hassle).
Beware that 200W is quite a bit below the Pro PSU's capability, though.
Best of luck to your project!
So it's already well suited to semi portable use i.e. cars, caravans, boats?