I did a little googling, and still find the answers are "logical replication, which is slow for large DB's" or "pg_upgrade, which requires nontrivial downtime".

I did a little googling, and still find the answers are "logical replication, which is slow for large DB's" or "pg_upgrade, which requires nontrivial downtime". Gitlab wrote a much more in-depth rundown in 2020 of a pg_upgrade, which in their case took 2h of downtime and 4h total time: https://about.gitlab.com/blog/2020/09/11/gitlab-pg-upgrade/ https://about.gitlab.com/blog/2020/09/11/gitlab-pg-upgrade/ (submitted to HN here: https://news.ycombinator.com/item?id=29926316 https://news.ycombinator.com/item?id=29926316). They included a video of the whole thing which is pretty incredible.