Thanks for your reply !
In the meantime, I baked myself a solution for that same problem that only relies on Bubble’s expressions, not other plugin or javascript required.
Here’s how:
My case here concerns the line items object on Stripe Checkout Sessions but it’s exactly the same process for other list of objects that must be passed as url-encoded.
in the API Connector, I use JSON Dynamic Value to set the query string entirely that I can then modify in the workflow interface.
In the workflow I use a list that contains Basket Items (with Product, Price, Quantity) and format it as text as shown below to create the query string.
I hope this approach can be useful to some of you guys. 
Happy Bubbling