The exact firmware from which Amazon deployed the new KFX DRM might be a bit tricky to identify, but the latest firmware in January 2023 was 5.15.1. I think it should be safe to update your PW3 to the last officially supported version, 5.16.2.1.1; it is a stable version which works with all the original hacks (after this version it gets complicated). If you don't like it you can always
downgrade to a lower version.
You can also try with a rooted Android device, but it can be hit and miss depending on the version of the Kindle app supported by your Android release. A related approach is to use an Android emulator; by using a specific emulator for a specific version of Android, it is possible to download books with weaker DRM. You can find the details
here.