> much less because the OS in ROM would write itself into RAM on startup and take about 20KB away.

RAM shadowing of the ROM did not exist in the Atari's (at least not in the original 400/800 models). The ROM's simply were physically connected to actually "be" the top 16KB of the 6502's 64k max address space. The CPU executed the ROM code by directly reading the code from the ROM chips.

Which is also the reason the original 400/800 models were limited to 48k max RAM. 16k of the address space was already used by the ROMs.