Before you go through this work, you know that there are existing datasets, right?

See: https://github.com/EBazarov/nsfw_data_source_urls and https://github.com/alex000kim/nsfw_data_scraper for example.

If you want to train a NSFW classifier, use the existing sets first. And use a pre-trained Imagenet classifier first and fine tune it. This will get you 90+% of the way there. It would make sense for you to have your own testing set to make sure that it works for your use-case (CVAT or VoTT work fine), but goodness, don't start from scratch.