Nice. So like you’re saying, as long as you personally are willing to store the other files you can definitely roll together as many as you want. Just hash the whole lot of them together and submit the hash.
Frees up onchain space but takes on-your-hard-drive space
This makes the scale limit not the onchain fees, but rather how many people want to go in with you and Joe many files you want to store indefinitely for each proof stamp you need.