Quick and dirty decoupled CiviCRM with cron and export/import?
I think I have a partial answer—or at least, a tool that's looking good.

Answering Demerit's comment: both the Backdrop/CiviCRM and D8/Open Social sites are going to have public pages. My original assumption was that D8 would be primary site, with CiviCRM information fed into it—but the last few days of deeper dives into CiviCRM indicates that so much functionality is available there "for free" that I'm likely to make that the primary site, with a secondary community.mydomain.org domain for the social networking aspects. There's just a ton of stuff I thought I would have to custom-build which someone has already included in core.

Still the case that I need a new user/CiviCRM account to create its equivalent on D8, and that single sign-on would be nice. Both domains are subdomains of mydomain.org, so I can tackle that with cookies, but since I have to do account creation anyway, a more elegant approach that allows new domains to be used would be nice (we currently own four). This YouTube video on data integration led me to Pentaho, which looks excellent both for its abstraction capabilities handling any dataset, as well as its scripting interface. Not seeing much point in going through the trouble of learning and implementing SAML if I can do it all in one package. (And for the initial release, there are few methods I could use to avoid all of this—i.e., creating a hash on the primary site that someone has to use to create an account on the second manually, and suggesting they log into both with the "remember me" checkbox turned on.)