Extremely common problem and largely represents the 'state of play' at a great number of companies, especially though that have had a software offering for more than 10 years.
Moving from manual testing to automation is frequently misunderstood. The target is assumed to be 'automating the manual tests' and when there is just that understanding, chaos ...
If you are familiar with Selenium, there is Winium that might be interesting:
Winium is a Selenium based tool for testing and automating desktop applications on the Windows desktop. It is easy to use for those who are familiar with Selenium.
It is practically Selenium for Windows.