> I don't want to create an account on the X instance of the fediverse, I want to create an account on the fediverse. It should be one big fediverse, which instance I use should be completely transparent and irrelevant.

This is the fundamental tradeoff. What you are asking for cannot be done and still be federated. Sorry.

The problem is that "federation" has only a small technical component; the majority of the "federation" problem is social not technical.

The social problem is "Bad actors exist. 1) How do I identify them? and 2) How do I extend or revoke trust?"

Even email, which everybody holds up as "federated", hits this problem and defers to centralization. For email, we anoint the DNS records as the primary repository of "identity" and are what are used to extend "trust" via DKIM, DMARC, and SPF.