Onyx is the best Android-based ereader company that I know of.
https://shop.boox.com/
I've had 2 of their ereaders and liked them.
https://shop.boox.com/products/gocolor7 is their latest 7 inch dvice which I think is the right size for manga? The older black and white devices 7 inch and 10 inch devices might be cheaper but will probably have older Android versions, so you might not be able to install some apps.
All eInk devices will be slower than LCD/OLED devices, because the display technology is different. There will always be a slight delay on page turning or pressing an icon because the particles have to actually move to change the display.
https://en.wikipedia.org/wiki/Electronic_paper has more details if you want.
EInk has other major advantages over LCD and OLED - better in bright light, less eyestrain, less power use. But the tradeoff is it's not quite as immediatley responsive to a finger tap.