Is it possible to encrypt with Veracrypt root partition of already installed Ubuntu without data loss? I think I need to use boot flash cause impossible to encrypt mounted root partition, right? Do I need to create separate unencrypted /boot? Any experience with it, trusted manuals, working and tested links?
Nope, sorry buddy, at this time Veracrypt does not support bootable Linux drives. You can, however, use it for all your external drives, no problem. You can find it on their web page (https://www.veracrypt.fr/en/Supported%20Systems%20for%20System%20Encryption.html). I suspect no-one answered you because it says so right there.