I'm currently writing an agenda app inspired by the Psion 3a/5 Agendas - because they were the best calendar apps I ever used.

Github: https://github.com/semiprime/pygenda Demo video: https://www.youtube.com/watch?v=uvQqFmlZ6nM

If you liked the S5 agenda, I'd appreciate any feedback you could give. (Video shows it on a Gemini, but I also run it on my Linux Laptop. Running on Windows is probably possible, but I haven't tried it, so you'd need to do some fiddling - in particular installing GTK3).