Quick notes on what I've learned from the advice on here and some of my own research:
* A lot (most? all?) of CUs have membership requirements of various sorts; sometimes that involves e.g. donating to a particular charity, which could be a deal-breaker if you're not into that charity
* There doesn't seem to be much published in terms of comparing CUs. NerdWallet has some lists comparing them, but it is unclear to me what their motivations and objectivity are.
* CU co-op shared branching is a thing (thanks @elplatt for this tip). It is nice to join a CU that participates in this, as it gives you more possibilities if you care about physical branch access. Locator here: https://co-opcreditunions.org/locator/
* From recommendations on here and online, https://alliantcreditunion.org kept coming up as a good option. For SF Bay Area folks, https://www.firsttechfed.com also keeps coming up as recommended.
* Didn't find much about retirement savings, yet.
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