I have found matte screen protectors to significantly cut down on glare and reflections that I'm able to adequately read in the shade on my iPad Mini 5. Yes, the brightness needs to be turned up a bit... a little over 50% for me. I don't PREFER to read on my Mini outdoors, but it is possible with a matte protector.
e-ink ereaders are not my first choice either. The screen refresh rate can be distracting, even on the most responsive ereaders. I prefer my Aluratek Libre Pro which not only has physical buttons for turning pages, but uses a non-backlight monochrome LCD screen. The brighter the ambient light, the sharper it looks and with none of the limitations of e-ink devices. But then again, I'm an oddball, so there's that.