Lots of open source designs that can route steel, like PrintNC [1] and OpenBuilds [2]. If all you need is Aluminium, then you can make do with something like the RS-CNC32 [3] or MultiBot [4].
Not parent, but I’d consider 0.001” over 10” to be totally acceptable for home and hobby use and 0.00025” over 20” to be acceptable for general production use (“great” isn’t really meaningful, but “makes parts to spec” is good enough).
That's the specification of a measuring instrument. Commercial VMCs don't hold 0.00025” over 20" even without being loaded. A slight temperature change will throw that off.
[1] https://wiki.printnc.info/en/about
[2] https://openbuilds.com/builds/category-list
[3] https://www.makerfr.com/en/cnc/rs-cnc32/
[4] https://hackaday.io/project/176110-multibot-cnc-v2