Not using "add to cart legacy", as I think this option was deprecated in newer versions of Hikashop (latest installed on my dev site):
For the layout, i'm using the "image and title" which currently has zero customizations now:
The result of this config + minor CSS changes:
To my knowledge, I have removed any customizations that could potentially be using the outdated "forms". The problem seems to be linked to a conflict between this plugin and the various hikashop default layouts. I can send backend credentials to this dev site if you would like to take a look.