Hello,
I am an expert in the same domain and perfect fit to update your Drupal 8.5.11 site into the latest Drupal 9 versions. according to your requirement and work efficiently. Since 2011, I worked on near about 150 + small to enterprise websites based on Drupal 6 to the latest Drupal 8x. Apart from drupal I have strong command on custom module development, contributed modules, AJAX, HTML, JavaScript, and CSS. Familar with github or BitBucket.
Your prompt response will be highly appreciated.
Thanks,
VJ
PS: I'll submit my best quote once SOW finalizes. No doubt, All payments will be payable by milestones. Please, keep in mind that payments happen ONLY after you are 110% satisfied with the project progress.