0 or 5 without decimal point

do you think you could implement an option to set when there is a 0.0 and 5.0 in the rating to only show 0 or 5?

I’m assuming you mean the in the rating summary?

Try this:

add_filter('site-reviews/summary/value/rating', function (string $value): string {
    if ((float) $value === floor($value)) {
        return (int) $value;
    }
    return $value;
});

it would be great to be able to display at 0,0 – i.e. if there is no rating – a text rating that there is no rating. We leave 0,0 displayed and this would be aesthetically great.

Try this:

add_filter('site-reviews/summary/value/rating', function (string $value): string {
    if ((float) $value === 0.0) {
        return __('No ratings yet', 'your-theme-domain');
    }
    return $value;
});

Display with text rating or separate text ratings for a predefined range, e.g.: 0-1 (poor), 1-2.5 (average), etc.

Try this:

add_filter('site-reviews/summary/value/rating', function (string $value): string {
    $rating = (float) $value;
    if ($rating <= 1) {
        return __('Poor', 'your-theme-domain');
    }
    if ($rating <= 2.5) {
        return __('Okay', 'your-theme-domain');
    }
    if ($rating <= 3.5) {
        return __('Good', 'your-theme-domain');
    }
    return __('Excellent', 'your-theme-domain');
});