Hi there, you might find it useful to take a look at the list of Mesh Overlay Network vendors listed on https://zerotrustnetworkaccess.info that’s the architecture you’re looking for. Good luck
I would suggestion OpenZiti (https://openziti.io/) if you want open source and NetFoundry (https://netfoundry.io/) if you want a commercial SaaS. They are similar to Zscaler Private Access but goes much further and crucially, supports both tunnelers which can deploy on the server, as well as SDKs which can be embedded into apps. Note, I work on both project/company.
This last part could be game changing as it sounds like (though not sure) you develop an application which you deploy at customer site. This would mean no additional SW, the ‘agent’ is effectively part of your application binary.