So the problem here is that cloud-init also looks for url= on the kernel command line 
Fortunately it looks for cloud-config-url= first, so if can point cloud-config-url= at a URL that just has an empty file at the end of it it should work. I didn’t test an autoinstall but an interactive install completed in a 2G VM and that probably uses more resources than an autoinstall. HTH!
(And we should find some way to not require these gyrations)