Thanks for clarifying. I can certainly see why that change would be problematic! I admit I am not a fan of the non-array configuration of flow variables for such nodes as it makes it very difficult, if not impossible, to configure them programmatically, as you are demonstrating.
The only thing I can suggest at the moment is searching the node repository for the deprecated version of the Joiner node, which is still available.
(I think you may have to switch in Modern UI for this search to work, if you are currently using Classic. You can switch back again after you have found the node and included it on your workflow)
search for:
joiner //deprecated
and make sure to choose the node shown in red below, as there have been several joiner nodes deprecated in the past!
