Ok I’ll test the dynDFE method.
I’ll report here my progress, so this post is to be updated 
1) find the zip file
Found two links for dynDFE v2, here and here. checked the md5sum, both files are the same.
# Disable Force Encryption V2
# By reikaze
# Based on Zackptg5's DFE script
On another link I found four versions for vayu, not sure if those ones are device-specific…
prin " Disable Force Encryption V3"
prin " For Android 13"
prin " By Kamisato Rei"
prin " Mod by alanndz"
prin " Based on Zackptg5's DFE script"
2) backup my device
As you read above I did a full disk image with dd.
I want also to test two apps that I read about on this forum
Namely Neo Backup and Swift Backup. I’d like to compare them to my old Titanium Backup.
3) flash the zip file
That’s where the fun will start…
… or not, the script failed 
.
[Image taken with my FP2]
Do I have to wipe before? (I was guessing first flash the zip, then wipe, but I may be wrong)
Is a factory reset enough, or do we have to select manually (several?) partitions to wipe?