> the 32-bit marketing started with the A1200

That was because the A1200 was the first Amiga to have a 68020 as the native CPU on the motherboard. The 68020 had 32-bit data registers and 32-bit address registers. Earlier Amiga's were designed around the 68000 CPU which was instruction set compatible with later 680x0 CPUs (which featured backward-compatible super sets). In the 68000's data registers only had 16 data lines connected externally, requiring two cycles to read or write 32-bits and the 32-bit address registers didn't have their upper 8 bits connected to external pins, limiting the directly addressable RAM to 16MB (24-bits). These compromises allowed the CPU to fit in a 64 pin DIP package while the standard 68020 came in a 114 pin PGA package and was fully 32-bit internally and externally.

However, it's confusing because the A1200 had a lower cost version of the 68020, the 68EC020, which also didn't have the top 8 bits connected and came in a smaller 100 pin QFP package. So technically, it had the same addressable RAM limit as the 68000 (although it had other instruction set and clock speed improvements).

Prior the the A1200 (1992) here was an earlier Amiga model, the A2500 (1989), which came with a full 68020 CPU but it was a 68000-based A2000 with Commodore's A2620 add-on accelerator card pre-installed, so it had both CPUs (although the 68000 was unused when the accelerator was added).