Thank you so much for your detailed and interesting answer, Irvinewade! After I read it I decided that, the best compromise for me (not expert), is to choose and use just a numbers password! Maybe it’d be a wise suggestion for every user at same tech level 
I hope, in the future, Purism will implement an ASCII full compatibility boot keyboard with logged in keyboards although, in my opinion, picture password remain the best option to unlock L5 (because digiting on the keyboard could be spied in crowded places (eg: trains, buses,…)! Hope Purism will implement it one day, at least as security option for us as choice!
Have a nice day 
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