Spec me a grinder/belt for finishing aluminium.

[Zeeflyboy]

Given they are fairly small parts with tight nooks and crannies I'm not convinced you'll be happy with anything other than sand blasting.

You could try tumbling in a finer abrasive material like aluminium oxide grit (Like often used for blasting), that's more likely to get into the internal corners etc than the large tumbling media. You might need to experiment a bit.