I recommend a dead man’s switch like Healthchecks.io, which can be selfhosted for free. Whenever you have something that’s regularly occurring, add an extra callout to your unique Healthchecks callout UUID as part of the automation, and Healthchecks will send you a notification if something misses its callout schedule. You can also attach whatever data (e.g. a log) to the callout so you can look back through the run history. IIRC Borg will give you a non-zero return code if it detects problems, so you can send e.g. https://hc-ping.com/your-uuid-here/$? and a non-zero code will signal a notification as well (more examples here).

Also, Borgmatic is really easy to use for managing Borg repos. There’s a lot of configuration options (including Healthchecks.io integration) but you can delete like 90% of it for normal usecases.