Could it be that the error is actually on the bank APIs? They are not actually returning a response in time?
As per the trace image, it states clearly that the request has timed out after waiting for 8 seconds. Try increasing it to say 100 seconds, and see if it still fails.
Another reason could be that your server is failing to connect to the bank remote servers, causing the request to timeout? But since you say that request is recorded on bank, this seems less likely.
From what I know, there is no special mechanism Wappler implements for API calls, so it should just work like any other API call.
Since you have been struggling on this for 3 months, I would suggest to build the alternate with Golang, and see if APIs also fail under load there.
P.S. I recently encountered an issue where API calls from Wappler to certain endpoints would just timeout in 0ms. Have a post in community, but since its a very weird issue, haven't received any reply. The solution I have implemented is to set 120 seconds as default timeout value in the core Wappler file from where APIs calls are made - worked well for me.