The way you describe it, it seems like they're a lot better and it might be more than setup. Apologies for the possibly stupid question, are you sure you're getting full brake input in the game?
Generally, things in racing games / sims that directly impact braking are:
- Brake pressure, which you say you've already maxed.
- Brake pad friction, but I don't think F1 2019 has this.
- ABS settings. Not a thing in F1 2019 though.
- Brake bias. Hypothetically speaking, as far back as you can go without being unstable under braking gives you the shortest braking distance. But I don't think this is going to shave a third off your braking distances even when optomised.
There's other setup options that indirectly impact it. If they have more downforce or are on softer tires, they'll be able to brake better than you because their tires are generating more grip. Likewise if their car turns in better (balance towards oversteer), they can enter the corner faster.
Do you trail brake? What are your cornering speeds like vs. theirs?
In any case, remember slow in fast out. Where you put your foot on the gas has a bigger impact on your lap time than where you put it on the brake.