I had the first Kindle in 2007 and just got the new Kindle Oasis for exactly this reason (transition from shorter form to longer form).
My one gripe as an Audible subscriber is that I wish it supported immersion reading (read and listen at the same time).
Another random service I love to use with my Kindle is Mercury Reader. I use it to send a subset of my longer form Pocket articles to my Kindle. I know there are a handful of solutions for this but I’ve found this extension to work the best.