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

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.