Zelle is a trade name of Early Warning Systems, which maintains databases of what you do in your bank account (checks paid and deposited, monthly balances, etc.). I would assume the banks that have native Zelle integration are required to become members and share this information anyway.
https://www.earlywarning.com/about
If your threat model is that you don’t trust your bank with your deposit account balance and transaction history, that is hard to mitigate. I don’t necessarily trust my bank, but I do know what regulators to call for recourse when they misstep.
Where can I find more info about the '8 months' estimate? I hoped RTP would be useful to me, but the only bank that allows consumer accounts to use their RTP network is Chase. I hope this new network also doesn't fall short of the promise.
Regardless, avoid PayPal and Venmo at all costs, as there are reasonable alternatives for many use cases. They have avoided regulation more traditional financial service providers operate under (banking regs), so using them is to invite suffering.