update moodle from 3.9 to the latest

As an experienced Moodle developer, I'm looking to work with someone to update my Moodle from version 3.9 to the latest version. Currently, I have custom plugins and themes installed, and need to ensure they are compatible with the latest version. I expect them all to be compatible after the upgrade, so the chosen candidate must have excellent knowledge of Moodle and the plugins/themes used. The chosen candidate should also be able to test all elements of the site after the upgrade to ensure everything is working correctly. If any unexpected issues come up, a plan to solve them should be provided. In addition to the upgrade service, I am open to the recommendations of the candidate for any changes that would help improve the moodle website.