Try the following code in a script - works for me on Tmux 3.1:

#!/bin/bash



# Create a new session named "newsess", split panes and change directory in each

tmux new-session -d -s newsess

tmux send-keys -t newsess "cd /path/to/directory1" Enter

tmux split-window -h -t newsess

tmux send-keys -t newsess "cd /path/to/directory2" Enter

tmux split-window -v -p 66 -t newsess

tmux send-keys -t newsess "cd /path/to/directory3" Enter

tmux split-window -v -t newsess

tmux send-keys -t newsess "cd /path/to/directory4" Enter

tmux split-window -v -t newsess

tmux send-keys -t newsess "cd /path/to/directory5" Enter

tmux select-pane -t newsess:0.0

tmux split-window -v -p 66 -t newsess

tmux send-keys -t newsess "cd /path/to/directory6" Enter

tmux split-window -v -p 66 -t newsess

tmux send-keys -t newsess "cd /path/to/directory7" Enter

tmux split-window -v -t newsess

tmux send-keys -t newsess "cd /path/to/directory8" Enter

# Set pane synchronization

tmux set-window-option -t newsess:0 synchronize-panes on

# Run command in all 8 panes

tmux send-keys -t newsess "npm run dev" Enter

# Attach to session named "newsess"

tmux attach -t newsess

For testing I'd recommend running pwd instead of whatever command you wish to run in 8 panes simultaneously (also to check directories are correct). ;-)

If you want to run different commands in each pane, you of course just run the command with send-keys after changing directory. But I was under the impression that you want to run the same command in each pane, and that's why pane synchronization would work.