I'm not really one for shilling my (decidedly unprofessional) coded projects, but submit that I have put together a shell script[1] that builds an agenda using date stamps from passed text files. The script is POSIX compliant if you pass a flag, but otherwise uses Fzf to filter and jump to entries. It's basically like a less extensible, but much shorter (in terms of lines of code) version of Org Agenda. I'm currently extending it to export an ICS file, which will allow automatic import into calendar services from Google, Fastmail, Apple, or whatever. Two-way calendar sync is beyond my will for faff, but if the spirit so moves someone I wouldn't stand in the way.

[1]: https://github.com/shushcat/txt-agenda