Attach to tmux session and run a command

It is time sensitive.

From the man page:

The target-session rules for attach-session are slightly adjusted: if tmux needs to select the most recently used session, it will prefer the most recently used unattached session.

Your shell alias is the way to go for this, it is not a workaround, but the way Unix/Linux works. Small tools to do small things to produce great results.

I think you can achieve what you're looking for by using an appropriate format with tmux list-session and parsing the output:

if [ -z "$TMUX" ]; then
    attach_session=$(tmux 2> /dev/null ls -F \
        '#{session_attached} #{?#{==:#{session_last_attached},},1,#{session_last_attached}} #{session_id}' |
        awk '/^0/ { if ($2 > t) { t = $2; s = $3 } }; END { if (s) printf "%s", s }')

    if [ -n "$attach_session" ]; then
        tmux attach -t "$attach_session"
    else
        tmux
    fi
fi

The format here for tmux ls gives, for each session, the number of clients attached, time when last attached (or 1 if never attached before, such as after tmux new -d), and session ID. The AWK script uses this information to find the most recently attached session with no currently attached clients and outputs its ID. We then either attach to that session or create a new one if no such session is found, like when the server isn't started or every existing session is attached.