the thesis will be that the free relays, to monetize, will sell users data... and paid relays would not...

That being said, even if the paid relay says it does not sell your data it may, you would be trusting them.

There are benefits thought, like higher limit to upload and host your media content and host all of your data until you end the service.

I suggest hosting your own relay to save your data from being pruned and erased as most free relays do since storage capacity is not infinite nor free.