This is a big part of why I dislike consolidating downloading and viewing.
I've been using PerfectViewer on a tablet for viewing for ages. I'm sure there's a better one and would be game for recommendations, but I am very used to this app and its quirks.
There exist any number of ways to download and sync the chapters to my device. I currently mostly use the mangadex-dl script and a syncthings folder and it's no trouble. And all my read chapters I can just move into an archive drive where I'll have them if anything ever happens.
This is standard practice with media. You use something like MPV for viewing and the downloading is handled elsewhere.