The initial_preferences json file can be used to configure the preferences that will be used by default in newly created profiles. The Brave browser is reading initial_preferences from /opt/brave.com/brave/initial_preferences. The Brave browser profile preferences are stored in ~/.config/BraveSoftware/Brave-Browser/Default/Preferences json file. You can use it as as a base and modify it to your needs for its use as initial_preferences file. You can run Brave browser in some qube, configure all the settings that you want, convert the Preferences json file to a readable format using this command: 

python3 -m json.tool ~/.config/BraveSoftware/Brave-Browser/Default/Preferences > ~/initial_preferences
Then edit ~/initial_preferences file and remove all unnecessary settings from this file (timestamps, UUIDs, default settings etc) and then use this file for your disposables. You can check if the resulting json file is valid by running this command: 
python3 -m json.tool ~/initial_preferences > /dev/null

Once you've created your initial_preferences file you can place it in disposable template: Open terminal of disposable template for Brave browser. Copy the initial_preferences file to the disposable template. Configure bind-dirs for initial_preferences file and copy the initial_preferences file to the right location by running these commands: 

sudo mkdir -p /rw/config/qubes-bind-dirs.d
cat << 'EOF' | sudo tee -a /rw/config/qubes-bind-dirs.d/50_brave.conf > /dev/null
binds+=( '/opt/brave.com/brave/initial_preferences' )
EOF
sudo mkdir -p /rw/bind-dirs/opt/brave.com/brave
sudo cp /home/user/QubesIncoming/*/initial_preferences /rw/bind-dirs/opt/brave.com/brave/
The same should work for any Chromium browser, but the initial_preferences file location and browser-specific preferences will differ.

Example initial_preferences config based on this recommended Brave configuration:

[details="initial_preferences"] 

{
    "brave": {
        "brave_ads": {

            "should_allow_ads_subdivision_targeting": false,
            "state": {
                "has_migrated": {
                    "client": {
                        "v7": true
                    },
                    "confirmations": {
                        "v8": true
                    }
                }
            }

        },
        "enable_media_router_on_restart": false,
        "enable_window_closing_confirm": true,
        "fb_embed_default": false,
        "google_login_default": false,
        "hangouts_enabled": false,
        "ipfs": {
            "resolve_method": 3
        },
        "new_tab_page": {
            "hide_all_widgets": true,
            "show_background_image": false,
            "show_rewards": false,
            "show_stats": false,
            "show_together": false
        },
        "rewards": {
            "badge_text": "",
            "show_brave_rewards_button_in_location_bar": false
        },
        "today": {
            "should_show_toolbar_button": false
        },
        "twitter_embed_default": false,
        "wallet": {
            "default_solana_wallet": 1,
            "default_wallet2": 1,
            "show_wallet_icon_on_toolbar": false
        },
        "webtorrent_enabled": false
    },
    "browser": {
        "custom_chrome_frame": false,
        "has_seen_welcome_page": true
    },
    "media_router": {
        "enable_media_router": false
    },
    "ntp": {
        "custom_background_inspiration": false,
        "custom_background_local_to_device": false,
        "shortcust_visible": false
    },
    "profile": {
        "content_settings": {
            "exceptions": {
                "cosmeticFiltering": {
                    "*,*": {
                        "setting": 2
                    },
                    "*,https://firstparty": {
                        "setting": 2
                    }
                },
                "fingerprintingV2": {
                    "*,*": {
                        "setting": 2
                    }
                },
                "shieldsAds": {
                    "*,*": {
                        "setting": 2
                    }
                },
                "trackers": {
                    "*,*": {
                        "setting": 2
                    }
                }
            }
        },
        "default_content_setting_values": {
            "brave_remember_1p_storage": 2,
            "cookies": 4,
            "httpsUpgrades": 2,
            "javascript_jit": 2
        }
    },
    "webrtc": {
        "ip_handling_policy": "disable_non_proxied_udp"
    }
}
[/details]