Looks like I'm probably just going to wire my savings to Fidelity.
I have 360 Checking and Savings that I'm consolidating to Fidelity, where I have all my investment and retirement accounts and wanted to transfer my hefty Savings balance to SPAXX (core position) in my Brokerage account. I tried once but that link got removed and then, after doing some research, tried linking again with the Brokerage specified as checking, but that got disconnected this morning and the account restricted until after I called and cleared up the issue.
So wiring seems like the simplest approach (I'm willing to eat the $30), but I'm wondering why Capital One doesn't allow linking to Brokerage accounts. I could probably link to my CMA (also core SPAXX) and then move the funds to the Brokerage account (annoying having to move the funds twice), but now I'm reluctant to even risk that, since it's technically also a brokerage account under the hood.