https://stacktobook.com/blog/substac...ethod-compared I use stack to book to download an archive of the substacks I read once a month and manually put it on my device (I have not tested if it works with paid content). It's not set it to forget like the above solution, but there is no limit and it's <5 min of effort a month
IIRC Substack and Kindle support RSS so you can try that also
The economical long-term solution would be to jailbreak and install koreader/wallabag/some calibre integration. I'm doubtful that this is something you'll genuinely consider, but the option exists.
The consumeristic long-term solution would be to get one of those android eink devices, which pretty solves any export or user experience issue for free given the flexibility of the software, provided that the user is willing to investigate