Mat, sorry for the delay, but I needed time to do a few more tests. Im going to try to put all the information in this post.
DHxHR_HQX is available in the 20201030 release. In the render window File Format FFMPEG, in the small box select qt from the dropdown list. Click the video spanner icon and in the compression window dropdown list select dnxhd qt, click Video Options View, highlight profile, click the profile button and select dnxhr_hqx.
The Use HW Device window is blank, I have never needed to change it. I set it to vdpau to test, but it made no difference. It is the same in both versions.
I have rendered DNxHR_HQX with both versions, I have been doing so for a while in 20201031.
I am importing 25fps 4:2:0 full range 10-bit F-log, all intraframe, HEVC at 400mbps, which I then transcode to DNxHR_HQX at 720mbps. I was trying transcoding smaller sections of movie to DNxHR444 at around 1700mbps which worked well, but when I tried importing the 92 transcoded clips the Appimage stopped well short and froze. I have a widget on the KDE panel that shows RAM useage and it was showing 55.71Gb. I then re-transcoded to DNxHR_HQX and tried again, it loaded them all but still filled the RAM and froze the system when generating proxies, the only escape is to press the power button and switch off. I temporarily set up a swap partition. It loaded the transcodes, but froze when generating proxies because it filled that as well, nominally 128Gb of memory!
Both times I transcoded in 3 sections, but it is necessary for all the clips to be on the timeline for editing. When I import the 92 transcodes to single user 20201030, the widget indicates 28.03Gb and there is no problem generating proxies.
DNxHR_HQX is 10-bit and yes, I am using RGBA-Float. I have tried various loading strategies but all show roughly the same RAM useage.
Edit: I meant to add that the 20201031 Appimage does not have the problem, but the unnumbered For Newer Distros and the 20210228 both do.