ht16k33 was just an example, note the "or similar" in my post - I don't exactly remember which holtek chip has individual brightness control, but there are some, they're installed in these commercial color rgb panels.

Yes, 2.5mm spacing, that is huge in 2.5 x 2.5 mm area, where you will have 4 RGB pixels, with SMT leds, you can have 3x3 or even 4x4 RGB pixels.