There is a parameter in the aircraft.cfg.
[brakes]
toe_brakes_scale=0.85
You could lower it for testing. I saw 0.82 on some other heavy and 0.71 on a prop commuter plane of mine. 0.60 on a 737.
I don't know how the actual setting is build, but lowering the value will surely lower the overall brake performance, and that's what you are looking for, right?

Are you braking with pedals or do you use the trigger or key? I'm asking because the already too high ground friction in FSX and the use of the trigger brakes easily renders the whole braking to be too good.

That's where the FSX flaw and the unnatural 'jump on the brakes' behaviour may come together since the trigger breaking always enables 100% force and the antiskid will receive a perfect friction setting, unlike to some real runways + tire combinations.
If you use pedals, you may step on them more gently to somehow simulate a firm but not too hard braking action for your passengers. That's what I do.

There's some payware out there to circumvent the FSX friction flaw and e. g. take runway conditions into account for all planes installed. I've never tried it though.

And whatever happens when tweaking the brakes, keep a smile on your face.  

Guess the toe_brakes_scale there.