I see no evidence of any attempt to improve parallax scrolling rather than the crippled 2x 8 colour dual playfield mode or 64 usable colours on screen or even a speeded up EHB mode to make it usable.
I'm not going to argue that ECS wasn't underwhelming, but you make it sound like improving parallax scrolling or speeding up EHB is just a simple hack on top of the existing hardware. It's not. 6-bit video modes on OCS/ECS (EHB, HAM, and dual-playfield) already use so much memory bandwidth that they have to steal half of the memory cycles from the CPU for bitplane fetch (when the CPU is accessing chip RAM, that is.)
If you locked the CPU out of chip RAM entirely you could get eight bitplanes' worth of memory access and have a 4-bit dual-playfield mode like AGA, but then you're stuck with only accessing chip RAM in vertical blank - you couldn't update the screen, work on an offscreen buffer, or even load in new audio samples during the frame (and God help you if your program was actually executing from chip RAM.) That works for consoles like the Genesis because they use tile mode, but the Amiga is a pure bitmap machine, and that'd just be unworkable.
As for "speeding up EHB," what you'd want there is a faster blitter - which you wouldn't get, because the blitter needs those cycles that are being taken by the extra bitplanes just as much as the CPU does! Whatever its other shortcomings, AGA had the right idea with doubling the chip RAM bandwidth (for bitplane fetch, anyway) - it seems that some of the other proposed chipsets were going to take it even further, though.
Point is, this stuff doesn't just happen by designer fiat. It's plenty easy for us to see these things in hindsight (and it was probably pretty obvious to the people on the ground back in the day, too,) but the engineers had to deal not only with the challenges of improving an existing design without breaking compatibility, but the challenges of working around management (who care more about financials than usability) and marketing (who wouldn't know what a bitplane was if you shoved it in one of their assorted orifices.) Considering those limitations, it's a bit more forgivable that ECS was just OCS with some of the arbitrary limits taken off.