You need to free up enough memory to do a simple downgrade. Sync3updater: MY2020+ cannot reformat or you will brick your APIM. Use Sync3updater to prepare a USB stick for version change.
To free up enough memory, you need to do a hard reset (let it reboot fully) and then two soft resets (allow to reboot after each one. Then immediately put in your downgrade USB stick that you made with Cyanlabs Sync3updater, just like you would if you prepared the USB from Ford. Note: this small freed up memory will fill up again quickly so do not think that alone will be sufficient as a fix.
Cyanlabs Sync3updater has an interrogator function to see if rwdata is full. Run the Sync3updater, go to the tool tab, select the usb drive, install the interrogator on it, put the usb stick into your car, wait for it to install and reboot itself (Sync)
with message that usb stick can be removed, go back to Sync3updater and read the file that is on the USB stick, see what the rwdata line says. If full will say 0M / 1.2G free memory for rwdata. The MR + SR + SR detailed above will free up about 5M. Normally free rwdata: 500-900M.
Tips for Sync3updater:
-never choose to force an install to bypass MY2020+ protection (ie. never choose reformat) if the software pops up with a warning; it means your selections are wrong and you need to fix it.
-never install maps with Sync3updater for a MY2020+, select "keep existing maps"
-for the purposes of the downgrade try 22200 or 21265, selecting app only (deselect the voice and gracenotes packages)
-use a trusted USB stick, preferably with activity LED
-make sure your Sync configuration is correct, ie. North America, navigation, current Sync version (ie 22251).
-if you still do not have enough memory for the downgrade, Sync will report failed install and list "version_err08". Next time: try doing master reset with 3 soft resets then USB stick. if you try to master reset and it hangs, try a couple of soft resets before the master reset.