The pi imager failing to flash and verify the image is reason enough to bet on a bad sd card or a bad download of the image.
On the second half of the following post I described some steps on how to verify those things: Anbernic 40xxh woes - #2 by bgelmini
Just be aware that you need to check the sha256 hash for the 35xxPRO in the MustardOS site, as the one in the post is for the 40xxH.