Crashing applications and system instability after an OS update can be due to several factors, including compatibility issues with the new OS, corrupted system files, or even hardware-related problems that are exacerbated by the update. Here are some steps you can take to troubleshoot and hopefully resolve the issues on your MacBook Pro Retina Mid-2015 running macOS Sonoma:

1. Check System Logs

• Console App: Use the Console app to check for any system logs that can indicate what might be causing the crashes. Look for repeated error messages or warnings that correspond with the times of the crashes.

2. Update Applications

• App Updates: Ensure all your applications are updated to the latest versions that support macOS Sonoma. Incompatibility between the new OS and older app versions can cause issues.

3. Safe Mode

• Boot into Safe Mode: Restart your Mac and hold the Shift key as it boots to enter Safe Mode. If your Mac operates fine in Safe Mode, the issue may be related to third-party software or extensions.

4. Monitor Resource Usage

• Activity Monitor: Keep an eye on the Activity Monitor to see if any application or process is consuming excessive CPU, memory, or disk resources.

5. Test User Account Settings

• New User Account: Try creating a new user account and operating from it for a while. If the problem is confined to your old user account, the issue might be related to user-specific settings or preferences.

6. Run Apple Diagnostics

• Hardware Test: Use Apple Diagnostics to check for any hardware issues. Restart your Mac and hold down the D key as it boots to start the diagnostic.

7. Reset NVRAM and SMC

• NVRAM Reset: Reset the Non-Volatile Random-Access Memory (NVRAM) by shutting down your Mac, then turning it on and immediately pressing and holding Option + Command + P + R for about 20 seconds.
• SMC Reset: Reset the System Management Controller (SMC) which can resolve issues related to power, battery, fans, and other features.

8. Check for macOS Updates

• Software Update: Ensure that you have the latest updates for macOS Sonoma, as they can include fixes for stability issues.

9. Disk Utility

• Disk Check: Use Disk Utility to run First Aid on your startup disk to check for disk errors that might cause instability.

10. Reinstall macOS

• Clean Install: If none of the above steps work, consider backing up your data and performing a clean install of macOS Sonoma.

• Apple Support: If the problem persists, it might be a good idea to contact Apple Support or visit an Apple Store for professional assistance.