You can trade Coinbase shared pre-IPO on FTX (unfortunately, not available to US users). The stock trading infra seems weird but makes sense under the hood and is reliable (happy to answer any questions people have).
https://ftx.com/trade/CBSE/USD
(this was mostly informational, but if anyone actually decides to use the platform and wants to get 5% off trading fees, also happy to share a referral link)