Basically there’s Kobo and Kindle where I’m at that are good. For me I like a bit more premium stuff in terms of having decent build quality etc and that means it’s either Kindle Paperwhite or Kobo Libre 2 that fits me. Both have proprietary shit software and their own bookstore but both can be side loaded. I generally prefer buying tried and tested hardware even if there probably exist some niche fully open source device out there but they tend to cost a lot more due to lacking scale of production. I wouldn’t however buy either of those new. I’ve setup reminders on second hand websites in my area and waiting till I find one in a good condition/price. Where I’m at that means for a almost unused device it should be about 10-20% below lowest historic price new. And older models around 50% below cost of a new model, naturally with a lot of variance depending on condition etc.