I've read a couple of threads from people who faced the same issue as I did, so I dug into the problem and found a workaround. I posted a shorter version of this guide as answers on the Discussion page and on Discord[discord.com].

Problem:
Since the "B" button is used for the Pause Menu as well as for the Parking Brake you always end in the menu if you want to stop your vehicle from rolling.
Unfortunately, there is no option in the game to reassign the button for the Pause Menu.
The most common solution is to reassign the button for the Parking Brake, but the HUD always states to press "B" for the Parking Brake - which sends you direct to the Menu.

Solution:
You can solve that issue by editing a single file without the need of any additional programmes.

In this guide I show you how to assign the Pause Menu to the more common "Start" button on the Xbox controller. Should work for PlayStation controllers too but I haven't tried that.

Editing files can be dangerous! Even though here it's just copy and paste of a single name you should always make a safety copy of the file you're editing before you change anything!

I'm not responsible if you mess up your game!

After I suggested my workaround to Cpt. Gummy♿, it turned out that he had already developed his own method.
With his approval, I have incorporated it into this guide.

I have therefore divided the guide into two segments.

• In part 1. "Controller Settings" I show the (simpler) method according to Cpt. Gummy♿
  
• In part 2. "File Editing" I present my workaround solution with changing files

I hope everything is clear and understandable. If you find any mistakes, please let me know.

(And please excuse my bumpy english ;))

• Click on one of the highlighted Buttons to view controller settings

• A new window is opened which should look like this screenshot below
  
• Click on "Edit Layout"

• After you've clicked "Edit Layout" the window now should look like the screenshot below
  
• To Change the function of the "B" button klick where the arrow is pointing

• Now you have to choose the function of the "B" button
  
• The top row lets you choose between various sources but since the Parking Brake is normally bound to the spacebar you klick "Keyboard" as source for the input, as highlighted below

• On the now shown keyboard just click on the spacebar

• After clicking the spacebar you'll get back to the screen where you see the function of the buttons
  
• If done right the row with the "B" button should now show "Space Key" as shown in the screenshot below
  
• By clicking "B Back" in the bottom right corner three times you'll get back to the OoO library entry
  
• Your changes are saved automatically

• Thats it - now you've rebound the button and using the parking brake won't open the Pause Menu

• The file we're searching is located in the "SaveGame" folder.
  
  
• Search your SaveGame folder on your system, which is typically located under:
  
  C:\Users\[Your Username]\AppData\Local\OutOfOre\Saved\SaveGames
  If you can't see the "AppData" folder it is probably hidden by your Windows settings.
  
  
• In the "SaveGame" folder you'll find another folder “InputControlls”, where the file we're looking for is located.

• Inside the "InputControls" folder is just a single file named "CustomInput.json"
  
  
• Open the file by using the standard editor

• There is a lot to scroll through so just use [ctrl] + [f] to search for the entry “Pause Menu”.
  
  
• The text should look like this:
  
  "(TagName=\"Input.Character.Pause Menu\")": ← SEARCH WILL BRING YOU HERE
  {
  "mappings": [
  {
  "preset": "CONTROLLER",
  "deviceName": "CONTROLLER",
  "devicePath": "",
  "connectionIndex": -1,
  "deviceTag":
  {
  "tagName": "Input.Type.Controller"
  },
  "triggerTag":
  {
  "tagName": "Input.Trigger.Continuous"
  },
  "inputType": "Button",
  "inputKey": "Gamepad_FaceButton_Right", ← CHANGE THIS
  "inputRange":
  {
  "x": -1,
  "y": 1
  },
  "outputRange":
  {
  "x": -1,
  "y": 1
  },
  "bDeadZoneFromStart": false,
  "deadZone": 0.10000000149011612,
  "scalar": 1,
  "bNoModifier": true,
  "bShift": false,
  "bCtrl": false,
  "bAlt": false,
  "chordKeys": [],
  "antiChordKeys": [],
  "antiFunctionTags": []
  }
  ]
  },
  
  
• All you have to do is to change the inputKey from
  
  
• Gamepad_FaceButton_Right
  
  to
  
  
• Gamepad_Special_Right
  
  
• The segment now should look like this:
  
  [...]
  "inputType": "Button",
  "inputKey": "Gamepad_Special_Right",
  "inputRange":
  [...]
  
  
• Now save the file and et voila the Pause Menu is bound to the "Start" button

• If you took a look to the key bindings before you've made the changes you may have noticed that the inputKey Gamepad_Special_Right was previously assigned to the in game shop
  
  
• You may change that as you wish in the game, because no further editing of the CustomInput.json is needed for that

The CustomInput.json uses the Key/Gamepad Input Names of the Unreal engine
So looking just for "B" button or something like this won't work.

Here's a link to a list of the input names used by the engine

https://michaeljcole.github.io/wiki.unrealengine.com/List_of_Key/Gamepad_Input_Names/#Key_Name_List

With that you can basically change every assignment on the controller within the file even though I'd highly recommend doing this only to change the button for the Pause Menu and do the rest in game.

And as I mentioned above - you do this on your own risk!