For me, when I got an iPad mini 2 many years ago I quickly stopped using Kindles and started using iPads and iPhones with Retina screens for 95% of my reading. I have the older basic Kindle (without a built-in light), but it mostly collects dust. The only thing it is used for is occasionally reading while outdoors in bright sunlit areas. Otherwise the Kindles (and all other eInk devices) are inferior IMO.
As far as speed, I don't have a mini 5 yet, but my iPhone X
R has the same processor and it blows away my iPad Pro 9.7" on speed! So the mini 5 should indeed be very fast compared to older iPads. I still hope to get a mini 5 some day, preferably before they are obsolete.
BTW, yesterday I checked the value of trading in my iPad Pro 9.7" on a mini 5 and it has dropped in value from $189 when the mini 5 was announced to $171. WTF Apple! I would likely sell it on eBay if I decided to trade up, although I hate eBay with a passion, or just keep it as the sound is great on the Pro.