> but it bothers me that tmux sessions share the initial environment the first session was launched from.
It does do this, but I am curious what would be better? I heard this complaint before and It baffles me. let me explain.
environments are... sticky, a child process inherits it's parent processes environment. so the tmux server starts and inherits the environment from the shell that started it, then each pane inherits the environment from the tmux server. As far as I can tell you want each pane to inherit the environment from somewhere else but can't figure out why or how that would happen.
My best guess is that you want tmux-attach to stash the environment it was ran in so that tmux could then use that when making new panes. my second guess is that you want tmux to use the current pane as the parent process for a new pane. both of which sounds like it would be even harder to control the tmux environment.