Pages missing after Joomla upgrade

We had a 3.x.x. Joomla website in the past which we have upgraded to a Joomla 4 website. The files have been moved from an old hosting company to a new one. The website had 270 pages in which we were able to transfer most of them when we upgraded the Joomla 4 but a lot of them have gone missing because the plugin we used to transfer the data had a malfunction or did not support the full capability to transfer the files. We thus have a huge number of links from the website that are missing or are going to 404. We think this is the reason, the ranking of the website has dropped from first page to second and this is detrimental as website results on the second page are pretty much not seen by anybody. We are constantly getting website links from other third party companies informing us that they are going to 404. These pages may not be recovered but in terms of SEO, we could redirect them to existing pages on the website or inform the search engines that they are no longer present. We are looking for a Joomla website and SEO who could check and fix the missing links and perform the aforementioned tasks and also check what had caused the ranking to drop and help recover the ranking as it's been on the first page for a long time.

Requirements:
- Experience with Joomla and its different versions
- Ability to troubleshoot and identify the cause of missing pages after an upgrade
- Knowledge of Joomla backup and restoration processes
- Familiarity with website functionality and the importance of missing pages

The freelancer needs to:
- Access my website to determine the version of Joomla used before the upgrade
- Assess the importance of the missing pages for the website's functionality
- Find a solution to restore the missing pages, considering that there is no backup available

Please provide examples of similar Joomla projects you have worked on and your approach to resolving similar issues.