Yeah, I can create files in ~/.config/environment.d/ to set some environment variables. My problems are that (a) that syntax only allows very simple variable as

Yeah, I can create files in ~/.config/environment.d/ to set some environment variables. My problems are that (a) that syntax only allows very simple variable assignments, not complex things like "run ssh-agent and add the environment variables it prints to stdout" and (b) those environment variables aren't available to the Wayland compositor or applications it launches.