Crashing upon startup (Mac)
Are you shutting down all non-essential programs before starting the game, especially anti-virus and any backup programs that may be monitoring the Documents folder?
Try verifying local files: in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Files...' button. Also in the Local Files tab, click the 'Browse...' button, then exit out of the Steam client and try launching the game directly from the app file in Finder. If you are using the GOG version, in the Galaxy client, select the game, then the settings icon at the top right, beside the Play button and under Manage Installation, select 'Verify / Repair'. That menu is also where you can select 'Show folder' to try starting the game from the app file.
Try doing a clean boot and then test the game. Start/restart your Mac and hold the Shift key down as soon as the progress bar comes up in the startup window, releasing it after the desktop appears. Alternatively, from the login window, hold the Shift key when you click the Log In button, and release it when you see the Dock.
Disable Steam cloud support either globally or for only this game. For disabling Steam Cloud globally, within the client click on the Steam menu and select Settings, and then Cloud. Or just for this game: go to your Steam library and right click BG3 and select Properties, then switch to the General tab and check the Steam Cloud section. Alternatively, exit out of the Steam client, or Galaxy for the GOG version, and just start the game directly from the executable when required.
For the GOG version, in Galaxy cloud support can also be disabled either globally by clicking on the gear icon in the top left corner and selecting Settings, and then Game Features. Or for this game: with the game selected, click the settings icon at the top right, beside the Play button. Under Manage Installation select 'Configure...' and switch to the Features section. Alternately, exit out of the client and start the game directly from the app container.
Next, try browsing to the '~/Documents/Larian Studios' folder and rename the 'Baldur's Gate 3' subfolder. This folder contains the saved games, configuration files and a level cache folder. Deleting or renaming it will get the game to recreate it on startup; playing the game from a different OSX user account would effectively do the same thing. With Steam running and cloud support enabled, the client would just download the cloud copy of your existing profile. If this is a new install and you don't have any saves, you can just delete the folder, and it doesn't matter if cloud saves are enabled.
After that, see if you can start the game. If so, create a new profile and maybe check that you can start a new game and save/load. If that works and you have any patch 8 saves, exit and copy a couple from the renamed folder into the newly created profile's /Savegames/Story folder. If that continues to let you start the game and load the saves, move the rest of the saves over.
If the game still crashes, delete the new Documents BG3 folder and extract the replacement folder from the download below into your '/Documents/Larian Studios' folder, and see if that will let you start the game.
The graphicSettings.lsx file is set to 1280x720 Windowed mode, Very Low quality preset, which you can change in the options either manually, or by clicking autodetect. See if this gets the game to start.
If the game still crashes, delete the replacement BG3 folder and if you have any saves that you wish to keep, rename the original back again.
If that doesn't help, please reply with your full system specs (you can use the 'System Report' button from the 'About This Mac' Apple menu item, or the 'System Information' app in the utilities folder), including the OS version number. Also include a crash report (collected in the 'Console' application, under 'User Reports'). Alternately, the crash reports may be saved in the '~/Library/Logs/DiagnosticReports' or '~/Library/Logs/CrashReporter' folders.
Mac