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Yes, it is entirely possible to upgrade from 8.anything to 9.anything.
However, you mention a database error with H2. H2 is not supported, it is intended for quick demos and temporary development and test systems (I've never run an H2 based system for more than a week!) and Atlassian mostly don't test their code for it, so it would be no surprise that an H2 system fails on upgrade.
The best thing to do would be to revert to the backup you took before upgrade, run it long enough to get an XML export/backup of the data, then shut it down. Then create a new server on the new version (ideally on MySQL or Postgres, if you were not planning to move to Cloud), and import the data.
The other option is to migrate to Cloud from the old version. You don't need to be on 9.12 to migrate. The migration assistant works better and has more capability on 9 over 8, but it's minimum requirement is actually that your server is on 7.6. You're on 8.5 which is a very solid version to migrate to cloud. After hundreds of migrations with my team, my recommendation is to upgrade to 8.1+ before migrating. It can work well from 7.6, but Atlassian cracked it at 8.1, and then when you get to 8.4+, a lot of the app's data will start to work as well. 8.5 is a good starting version for a Cloud migration, there's usually little need to upgrade past 8.5
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