Wasabi is built to be 100% AWS S3 bit-compatible (same AWS API constructs for storage & identity management). No need to change your S3-compatible application when using Wasabi.
For what the error mentions, it seems to be related to access credentials as it mentions a 403 error. I'm glad you find the solution, which was the issue?
How you make your custom domain to resolve the external storage URL is not a Chevereto responsability, you have to check the Wasabi docs on how to map a cname.
Sorry, you have made it difficult for external storage. I use external storage on a few sites, but of course different themes, it doesn't work as functionally as you do.
I don't want the custom cname, I want it whatever it should be, and you don't serve as explanatory about it. This much cname etc on my other sites. I did not do anything