TYPO3 – V10

I am currently managing a website with a language set to German, hosted on domain.de (site ID is 5). We have a new requirement to introduce an Austrian variant of the site under domain.at. The goal is for the Austrian domain to display the same pages and content as the German domain, unless specific overrides exist in the new language for domain.at.

I attempted to implement this using language fallbacks by setting fallbacks: 5 and fallbackType: fallback in my configuration. However, I’ve encountered an issue: while the Austrian homepage (domain.at) loads with content of the (domain.de) if the page exists in (domain.at), pages which do not exist will have URLs incorrectly point back to domain.de/xxx.

I am looking for a way to configure an alias for domain.at such that it:

Displays content from domain.de when no specific content is available for domain.at.
Correctly resolves URLs and content paths to domain.at when specific Austrian content is available.
Could someone provide guidance or point me to what configuration or approach could best achieve this? Is there a way to set up such an alias domain configuration in TYPO3 v10?

Thank you in advance for your help!