Hi friends

Hoping for advice.
I am planning to buy ereaders for my kids to use for school. We homeschool and as they are getting older, many of the books we need for literature and historical fiction are available as free ebooks.
I really do not want to get Kindle Fires because I don't want them playing games and that is all that happens on friends' Kindle Fires - unless they're going to websites... which is also not reading!
But the benefits of an ereader are great and I think they would be an asset for school & family
Right now I'm collecting up books into Calibre and organizing them with tags, then copying them over to a dedicated Homeschool library. I would like to put this library onto each device.
I'm looking to buy 3-4 devices and have considered buying from ebay or possibly waiting for Black Friday sales (we have a 7" Windows tablet they can share til then, but I would prefer ereaders for reasons given above).
I order a lot from Amazon and we gave my MIL a Fire tablet as a gift a couple of years ago and she likes it... but not sure if this would be the best option. I've seen that a Kobo is good with Calibre-organized book collections.
Any suggestions on good devices - and also what editions might be good if I decide to buy from ebay to help us afford for multiple students (we have 7 kids!)?
Most of the books will be epub/mobi but I also do need them to be able to open and read PDFs for a few books available only in that format.