my only complaint with the pg_upgrade (with or without the --link) is for some reason, it does not move the statistics over, and you have to rebuild them, or have horrible performance for a while, until each page hits it auto-analyze thresholds.
I'm doing some testing now for my DB, and the rebuilding all stats takes far, far longer than the upgrade. The upgrade takes seconds, and it takes a while to analyze multi-TB sized tables, even on SSDs.
Yeah, there's some kludgey workaround for this that is definitely 80/20 kind of material...pg_upgrade will generate a script that does progressively more accurate re-ANALYZE so you're not flying your early queries totally blind. Maybe look into running that.
As for point #7, if your upgrade requires hours, you are holding it wrong, try pg_upgrade --link: https://www.endpoint.com/blog/2015/07/01/how-fast-is-pgupgra...
(as usual, before letting pg_upgrade mess with on disk data, make proper backups with pg_basebackup based tools such as barman).