Believe it or not, the Atari 2600+ multi-carts are made essentially just like the originals were in the 70's. They also work on original hardware, as do the controllers and paddles and vice-versa. Only the guts of the 2600+ itself are "modern", which is why it's still a glorified ROM dumper running Stella.The RockChip RK328 is the same SoC found on the Evercade unsurprisingly. I bet those cartridges also contain SD NAND chips on there too.
@esmith13 Maybe this will give you some good ideas.