I have been using a Sony DPT-S1 and a reMarkable 2 for this for the last 4 years. The Sony has a nearly perfect display and weight but only supports PDFs and requires manual content transfers (usb, microsd, or download via a local non-https web server) while the rM2 has excellent transfer mechanisms but has a smaller display. The Sony is quite old now (released 10+ years ago) but still has excellent battery life and functionality - you can probably find it on eBay for a very low cost.