Joomla Site Error Fix & Upgrade

My Joomla site (version 3.10.12) is currently experiencing critical issues. The administrator pages are showing the error "0 Class 'JFormFieldList' not found", preventing me from upgrading the site. I have various custom extensions and modules installed, which may be contributing to these problems.

What I need:
- Troubleshooting and fixing of the errors
- Upgrading the site to the latest Joomla and PHP versions
- Updating all my existing custom extensions/modules

Ideal Skills:
- Extensive experience with Joomla
- Proficiency in PHP
- Skilled in troubleshooting and fixing Joomla errors
- Experience with updating Joomla custom extensions/modules

Please bid only if you can provide the above services.