To my eyes, it seems flat.. but the problem is I'm wanting to do very small writing, 1.5mm height and line width of around 0.2mm. So I was thinking it may be a problem.

I've not surfaced anything with aluminium yet.. it's 3mm thick stuff so not a lot to play with.. What sort of tool do you use for that? I only have a standard surfacing bit described as "for wood"

The plan is to mill the aluminium.. then paint it.. then lacquer it.. then apply some vinyl signmakers paper on top.. engrave through that.. then paint it.. then remove the vinyl paper.. does this sound like a doable plan?