The stub secrets client is just a key->value API, so the value of a proxy is pretty limited. It's not a hard enough problem that anyone is interested in having a separate product for it.
Also, the "stub" client wouldn't really be a stub, as all the "ambient environment" secrets adapters would necessarily be local to the client rather than to the proxy. The client library would be a bit like using dnsmasq(1) as a local "stub" DNS resolver — where it reads your /etc/hosts and so forth, but for most things is deferring to a configured upstream DNS server.
Even this secrets manager proxy that the OP is about is explicitly to be used in legacy situations where you can’t use the AWS SDK, which is preferred because it does all of the stuff you mentioned for you.
However, this is Hacker News! If you think you see a problem that can be solved that other people don’t, why don’t you build it?