1280 SHRES mode and exploiting the fact analog CRT TV wont be able to display this and interpolate down to 160 pixels worth of ~80 colors. The key seems to be bad TV and sitting far away :) Only one bitplane is filled with actual color data, 1280 / 8 = 160, all other bitmaps are prefilled with special patterns, system bus is heavily saturated leaving very little for CPU.

This gave me different idea instead of smearing colors on TV. 2x RP2040 pico. One soldered on top of Hybrid/DAC where it accumulates digital data 8 clocks at a time and automagically translates planar to chunky, second RP2040 playing a role of DAC. Sprites not supported for obvious reason, need all the pixels pushed out to reconstruct flat chunky data. 2x pico because one doesnt have enough pins to fit everything, and fpga/cpld uneconomical in the face of $1 RP2040s.

OCS/ECS Hires 640x @16 colors would produce 320x200@256 with zero performance penalty out of A1000/500.

This is ~how Commodore A2024 monitor worked internally, except 23pin Video Connector had only 4 digital data bits resulting in stupid refresh rate. ~1000x1000 at 10Hz https://bigbookofamigahardware.com/bboah/product.aspx?id=863

Afaik Commodore Akiko chip was so braindead it took chunky data in and translated that into multiple planar writes to ram killing any potential speed advantage.