Just an update - I found this forum post that addressed the Health Check ‘failed to run background job’ issue.
Running zammad run rails r "Delayed::Job.where(failed_at: nil).each(&:invoke_job)", then systemctl restart zammad elasticsearch cleared most of these errors.
Emails that could not be processed: 31 < the only error that stayed.
However, after about 15 minutes, they all returned. Different quantities of times and attempts.