Id say firetab. They are cheap, they're gonna be on sale in a week or two during prime day. Cost a fraction of what an ipad does. I use one for reading comics and occasionally watching stuff on it. Works great for that purpose
We bought two FireHD 8 Kids, which come with a great case, charging over USB-C and the ability to put a MicroSD card. Considering how cheap MicroSD cards are, it made sense to pick a 128GB for each and download as many movies through Plex as we could into it.
I use comicrack, I have the fire10HD. I think its great. For Comicrack it takes a little bit of getting used to, but once you have it going I promise you'll love it, especially if you have a large library. I have like 8000+ comics, so much easier to read. Theres a pretty good workflow someone made on the comicrack subreddit. Note - comicrack isnt available on the play store, but the apk is on the subreddit