Display Stores by Region with relations
You could loop all regions and select their corresponding stores

{# you can as well include eager loading here #}
{% for region in craft.entries.section('regions').storesRegion(':notempty:').all() %}
    {% set storesForRegion = craft.entries.section('stores').relatedTo({
        targetElement: region,
        field: 'storesRegion'
    }).all() %}
    {{ region.title }}
    {% for store in storesForRegion %}
        {{ store.title }}
    {% endfor %}
{% endfor %}


If you want to increase your performance you can use eager loading at this point and include with(['storesRegion']) to your region Query