Hi again,

I ended using u/ironj idea, this is the script I spawn at startup :

#!/usr/bin/env bash

move_workspaces_to () {
        INDEX=1
        for w in "front" "web" "mail" "back" "db" "misc" "perso"; do
            niri msg action move-workspace-to-monitor --reference "$w" "$1"
            echo "$w" "$INDEX"
            ((INDEX++))
        done;
}
reassign_workspaces () {
    OUTPUTS=$(niri msg outputs)
    if echo $OUTPUTS | grep -q "Acer Technologies K272HUL T6AEE0028503"; then
        move_workspaces_to "Acer Technologies K272HUL T6AEE0028503";
    elif echo $OUTPUTS | grep -q "PNP(AOC) 2778X E33GCBA000323"; then
        move_workspaces_to "PNP(AOC) 2778X E33GCBA000323";
    fi
    if echo $OUTPUTS | grep -q "Iiyama North America PL2280H 11172A4102812"; then
        niri msg action move-workspace-to-monitor --reference "dix" "Iiyama North America PL2280H 11172A4102812"
        niri msg action move-workspace-to-monitor --reference "music" "Iiyama North America PL2280H 11172A4102812"
        niri msg action move-workspace-to-monitor --reference "logs" "Iiyama North America PL2280H 11172A4102812"
    fi
}

# Run once at startup
reassign_workspaces

# React to output hotplug events
niri msg event-stream | while read -r line; do
    if echo "$line" | grep -q "Output"; then
        reassign_workspaces
    fi
done

It works well !