Here's my understanding of the issue (in US law), though it may not be 100% correct...

We all have copyright to the things we create at the time of creation. It's inherent. The envelope trick is often referred to as "poor mans copyright" and while it may help establish some evidence, it's not actually very strong in courts as it's not very tamper proof. Time stamped computer files, website records, and certainly blockchain content would all be much better in my opinion!

Paying to register a copyright gives certain benefits. 1. It places the burden of proof on the other party. So if you sue someone for infringement you wouldn't have to prove they didn't create the work... they would have to prove they did. It basically gives you an advantage. 2. You have to have the copyright registered to collect damages. So if you've just done the envelope trick, or have your work recorded online and someone infringes your copyright, you'll have to prove they did, and will only be able to get them to cease infringement, but not necessarily pay you damages or any profits from the infringing work unless you've registered.

So I say get the work out there, let the world know it's yours, and if successful enough and you can afford the expense, consider registration down the line.