[Symbiote]

I would use the existing nodes (just like the SQL Server upgrade, you have reduced redundancy during the upgrade).

Under 1 minute of downtime was acceptable for us, so:

  systemctl stop postgresql-11 && \
    pg_upgrade --link ... && \
    systemctl start postgresql-12

was more-or-less the process last time I did this. We have only 500GB of data, and I think pg_upgrade ran in 15 seconds or so.

If a minute of downtime isn't acceptable, then it presumably isn't acceptable in case of unexpected hardware failure either, and you'd be using one of the commercial multi-master extensions.