> it feels like pretty much every stock broker app will now let you buy Bitcoin and they'll do it with lower fees than Coinbase

Coinbase even undercuts themselves. Use the Coinbase App - 2.5% fee. Use the Coinbase Exchange App (Coinbase Pro) - 0.5% Fee.

So... buy your Bitcoin (and others) through the Coinbase Pro website/app, then transfer them into your Coinbase wallet/vault for free.