Can you use Mosaico to create message templates
You can try the Mosaico Message Template Synchronisation extension.
From the README:

This extension enables you to use CiviCRM-Mosaico templates with scheduled reminders, personal messages, etc. It does this by automatically copying each template from Mosaico to a CiviCRM &quot;Message Template&quot;.
When you install the extension, all existing MosaicoTemplates will be automatically copied to MessageTemplates.
When you create or update a MosaicoTemplate, the corresponding MessageTemplate will be updated.

There's a few gotchas/caveats to look out for - see README on branch issue/6 for fuller explanation about what happens with things like unsusbscribe links which don't apply to a log of message templates.