My preferred choice wouldn’t be a chop saw, I have cut aluminium perfectly well (and accurately enough for mitres) with a compound mitre saw with a blade suitable for aluminium.
If it’s a one-of, I wouldn’t buy a cheap crappy mitre saw, I would hire a decent professional one. I would go to the hire shop and check out what they’ve got -DeWalt/Makita/Bosch/whatever, and see how smooth they run on their horizontal stanchions, how easy/freely they are to adjust – no need to plug them in. Although the default brand would be DeWalt – it’s just that I’ve known badly looked after DeWalt mitre saws to be a pig. You’ll need to buy an aluminium blade. IMO
EDIT : although I’ve said “see how smooth they run on their horizontal stanchions” you won’t need the pull-out feature for what you’re doing, it’s just that a mitre saw which runs freely on its stanchions is likely to be of professional quality and well looked after, and therefore far more likely to cut well and accurately.