I built this module which generates automatic srcsets for your Markdown images: https://github.com/jan-xyz/hugo-module-img-srcset
All you need to do to get source sets now is:
In your hugo project run
hugo mod get -u github.com/jan-xyz/hugo-module-img-srcset
and add the module to your
... theme = [ "github.com/jan-xyz/hugo-module-img-srcset"] ...
I hope this helps someone I was looking for something like this for quite a while and was actually pointed by @bep into the correct direction today: https://github.com/gohugoio/hugo/issues/5034#issuecomment-706602779