8K not opening in PE2020

Premiere Elements is not designed to work with 8K video or even 4K video at 120p. To edit this type of video you'll likely need a more advanced program like Premier Pro CC.

You have posted in the Photoshop Elements forum.  I am going to move your post to the Premiere Elements forum where you will get some more authorative advice.

I have the A1 camera and have shot some samples of each format of 8K video that it provides.  As you say, although PRE 2020 will open the files and downsample the 8K to 4K, I find the performance is too slow to do any meaningful editing work and I have a fairly powerful computer.  Nor will the video preview properly in the Organizer.  It is no better with PRE 2021.  

AFAIK, there are no current consumer level NLEs that will edit 8K natively.  I have a recently released advanced consumer NLE that supposedly "supports" 8K video, but it too simply downsamples the 8K to 4K. 

Bottom line - I don't think PRE officially supports 8K video (and almost certainly won't support 4:2:2 video).  But, hopefully, the experts over on the PRE forum can give you better advice than me.