Hello Tom713,
i think the problem is the used composer version.
The supported composer version for OXID 6.5 is Composer 2.2
You can get it with composer selfupdate --2.2
For older OXID versions as for example 6.2.4 / 6.2.5 the supported composer version is 2.1.6
I don´t know OXID would work with MySql 8, the recommended version in the system requirements is 5.5 / 5.7
All requirements are listed in the doc:
https://docs.oxid-esales.com/eshop/en/6.5/installation/new-installation/server-and-system-requirements.html
Kind regards,
Michael