Considering that a Pi CPU sill runs about 50 times as fast and has 100 times more memory than a NeXT had, whatever Unix you get running on it doesn't have to be "light" by OP's measure.
With a 25Mhz CPU and 16MB of RAM, every cycle and byte has to be accounted for.
I don't really think downloading a disk image and copying it to a microSD qualifies as a major endeavor.