Apple offers several layers of security around account. Unfortunately you were still using an older form which was poorly implemented and sometimes easily circumvented by social engineering. In 2015 Apple transitioned to using two factor authentication instead of security questions. Since then Apple has been persistently encouraging users to switch to the more secure system. Apple allows you continue to use security questions if you already use them, but if you end up having problems then they no longer get direct support.
If somebody has successfully taken over an account to the level you indicate then there is probably not much chance of recovering ownership. Have you tried contacting Apple and seeing if they have any other ways for you to demonstrate ownership (note that anybody taking over an account could also have removed any other information demonstrating this)?
This support page has information about contacting Apple support and service ➔ https://support.apple.com/106932 If live support options (chat and/or telephone) are available they will show near the bottom left.
- Telephone numbers may only work when calling from the same country.
- Chat options (telephone numbers appear as active links, not plain text) may depend upon cellular connectivity. If a number shows as an active link, click on it to see additional options.
- If chat is not available on the top level support page, try selecting your hardware and narrow down your issue to see if a support article offers a support link offering chat. For example, the contact support link on this page https://support.apple.com/119902#zerobalance offers a chat link about emptying account balance even on a computer where the top level page does not offer any chat.
With an iPhone, iPad, or Vision, you may be able to chat using the Apple Support App if it is available in your country —> https://apps.apple.com/app/apple-support/id1130498044