At least e-Ink screens are becoming cheaper and better so there'll be plenty of alternatives if they block side-loading.
I switched from Kindle to Boox. Boox eink tablet can still run Kindle the Android app, but overall it is much better device.
+1. I have two Boox devices and they're both excellent. One is a 10 inch reader that's quite thin, and the other is essentially the equivalent of an e-ink phone. Where to buy books is still a challenge for me, though. So far I've been relying on my decrypted library that I've been buying over the years, but now that Amazon closed that hole, I'm no longer buying from them.
What are the best alternatives for Kindle store?