Hi @Fredrik Johansson ,
as you already stated: The Activity Log details are deleted after 90 days. There is no option to extend/change the retention period.
The only option is to copy the Action Log details for long term archiving for instance in a Storage Account.
If the VM is created before the last 90 days there is no chance to get the detail of the creator of the VM. Sorry.
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten