Hello Max,
Unfortunately, that is not really up to us.
As you may have already noticed, those custom actions are developed using WiX Toolset, which is not our product.
As far as I know, .NET Core support for custom actions is still on their TODO list, as it can be seen here:
https://github.com/wixtoolset/issues/issues/6107
Additionally, they have some more issues, like the template not appearing in Visual Studio:
https://github.com/wixtoolset/issues/issues/6185
Although they have the v4.0 version released, they themselves do not advise updating just yet, as the new version may introduce new bugs.
https://wixtoolset.org/releases/
(Read under Development Builds section)
In fact, I have tested this just now and it doesn't work. I have installed v4.0 of Wix, updated my VS extension and created a v4 template for a custom action. If i select the project --> "Properties", I can not select .NET Core (even though I have manually installed .NET Core 6.0), I can only select .NET Framework up to 4.8.
Best regards,
Catalin