Wanted to add here that in order for Squirrel to work properly .NET 4.7.2 or similar needs to be used.
Custom events won't fire in .NET 5, neither in .NET 6: the App does not detect Squirrel Aware Assembly Attribute as well.
It's broken on a couple smaller details.
Took me the entire morning to figure this out, hope this helps for anyone having the same issues looking around.