Just my two cents: I have the HD 8 and HD 10, both the most recent 2020 and 2019 versions, respectively. For $100 the Fire HD 10 is quite good.
The lock screen ads are a minor annoyance, and you can pay $15 after the fact to remove them, or you can use an Amazon FireToolbox (look it up) to get rid of the Amazon experience and make it more like a generic Android tablet.
The battery life is excellent (the lock screen chews up almost no battery) and you get a 1920x1080 full HD display. I haven't noticed any sluggishness when surfing and reading eBooks or watching Amazon Prime or Netflix. The latest 2019 HD 10 is a considerable step up in computing power from the 2017 model; it's now an octo-core device and the cores are pretty decent. It's based on Android 9 now, which is pretty recent. I think the 2017 version was still running Android 5 behind the scenes?
It certainly isn't as good as a $329 iPad but it's less than 1/3rd the price. Keep in mind too the iPad is using an A10 chip, which is better than the cortex A73 inside the Fire HD 10, but it's almost four years old and an upgrade in the next iPad release is almost certain. If you're not gaming like mad the Fire HD 10 is a damn good device, and I say this as someone who resisted the Fire HD for years, using Barnes and Noble Nook HD tablets instead.
The Fire HD 8 is decent, only quad core and no full HD display but you can get one for $60 on sale. Both the HD 8 and HD 10 have fairly sturdy cases and aftermarket covers are plentiful.