Just use CALDAV; it's designed for making calendar entries automatically via email. I'm not hip with the fashion for putting an LLM into everything. I think it's lazy.
(Somehow I doubt whatever caldav software the above poster references takes more than a second to process multiple emails.)
I don't have a problem with a background task taking a minute or something...
But for ingesting unstructured data I didn't generate, I'm going to reach for an LLM now (provided latency isn't an issue).