Been a while... but after trial and error I managed to actually get something running in .Net8
There is also a separate Registration tool in here for the addin, so no need for regsvr32 or dscom.
https://github.com/HCarlb/DotnetExcelComAddIn

Works great on Win 11 x64, Office 2016 x64 with .Net8.
(but wont load if build on .Net9)