Legacy PHP CRM Upgrade

I have a twenty-year-old, PHP-based CRM running in Docker on Google Cloud. The core interface works well for my team, so I want to preserve the current look and feel while modernising the stack from PHP 5.7 and MySQL 7.0 to supported versions... (Budget: $5000 - $10000 USD, Jobs: API Development, Application Performance Monitoring, Backend Development, Docker, Google Cloud Platform, MySQL, PHP, Python, Software Architecture)