My hunch is this 
. Try to get rid of both Zram and XFS (one by one) and see what happens. With Zram as my first suspect. Discourse should run fine with regular swap and ext4. These optimizations might be fun but they are currently raising the complexity of your installation. Once your instance runs fine you can add them back one by one and see where things break.
As a general rule, try to stick as close to a recommended install first, then add your own smart stuff.