Fwiw, this is pretty much a slam dunk usecase for current LLMs.

Vibe code a script that parses your existing text file and creates events in your chosen calendar app. Then run this script on a schedule

Explicitly tell it to add a tag or anything else identifiable so it can Auto remove/update the events on changes etc.

You'll have a PoC in minutes and will likely be happy with the result within an hour, if you're using Claude Code