I think you are trying to simplify things when they’re not simple. Power isn’t just as simple as saying “how much output current is from cerb’s 3v3 pin”. If you connect the cerb to a 5v 200ma supply, then the limit will be probably be the input source. If you connect to a 1a or 2a supply, then the limit might then move to be the on-board regulator.
So you said earlier that the current draw (from spec sheet I assume?) is up to 100ma. The expected regulation capacity of the reg on board the cerb40 mk2 is 800ma, so if your current draw is only 100ma then the reg should handle that and the cerb40, if that’s all you have running.
Personally I’d try a phone charger (a modern one will supply regulated 5v usually at around 1a for fast charging) as your power source to see if that behaves acceptably, that should give you plenty of headroom before you approach the limits of the power supply.
The other thing I’d suggest longer term if you don’t have decent current measuring (tools or experience or capability) is getting one of these https://www.tindie.com/products/FriedCircuits/usb-tester/ and potentially the OLED backpack. Simple way to connect a meter and see what current draw is, or with the OLED you can just see the draw on screen over time. That may help you see if the RF module is actually drawing more than it’s meant to. I have had one for a few weeks and it’s easy to use and helpful to this kind of problem.