Thank you for your answer.
I did play around bbp_get_teeny_mce_buttons but didn’t get it to play nice.
I ended up using the full MCE and removed all additional editor rows and trimmed the first row.
Thank you @lynq for the initiative writing this up 🙂
If you like to show the sub-forum description you could use get_post() and modify the code from @lynq to fetch it.
Something like this:
$get_sub_desc = get_post( $sub_forum->ID );
$show_sub_desc = $get_sub_desc->post_content;
then add $show_sub_desc to the output.