FeedMe for Craft 3: Tweaking attribute values from feed before FeedMe process it
Have you tried EVENT_AFTER_PARSE_FIELD https://github.com/craftcms/feed-me/issues/516 

use yii\base\Event;
use craft\feedme\services\Fields;
use craft\feedme\events\FieldEvent;

Event::on(Fields::class, Fields::EVENT_AFTER_PARSE_FIELD, function(FieldEvent $event) {
    if ($event-&gt;fieldHandle === 'myAssetsField' &amp;&amp; $event-&gt;fieldValue !== null) {
        // $event-&gt;fieldValue will be an array of assets (already saved).
        // Modify the assets or the field value as needed...
    }
});