Solved No icons

Bootstrap 3 in Crypsisb4 : see screenshot.
 
-
Selection of Font Awesome : I have not this in the Helix admin. Same version 2.0.6. (2nd screenshot).

sorry maybe i'm overlooking it but i really can't find the option for bootstrap 3 in crypsisb4.

You have nothing overlooked, this option does not exist. We don't have a template in Kunena 5.2 that works with Joomla bootstrap 5 templates.
This is only available for Joomla 4.0 and Kunena 6.0, but there is no stable version of Kunena 6.0 yet. 
The combination Joomla 3.10 with bootstrap 5 template and Kunna 5.2 does not work, unless you find a third party Kunena template which is b5 compatible.

I think your website and extensions may not be quite the last .
my example is from Joomla! 3.10.4 with the shaper_helixultimate - Default template and Kunena Version: 5.2.8

I just updated Kunena to the last version (5.2.8 / 2022-01-01) and now, no more selection of bootstrap version in the field for the icons types.
Only Font Awesome. But no icons Font Awesome displayed anywhere. Not sure it's a problem for me.
The changelog of Kunena 5.2.8 contains :
Bug fix : Fixed icons disappearing in Kunena 5.2.7 (#8634)

My Joomla is also 3.10.4.

ok but in joomla do you really use this shaper_helixultimate - Default Template ?
and is the website online to view it?

okay can you join the kunena category as a test Category Icon: eg enter fas fa-atom and see if that works
got that from here when you click on start using this icon the correct code comes up
sample icoon code