For me it’s reminders. I use slack for work, and whenever anything comes up that I need to take care of (personal or work related) I’ll tell slack “/remind me about <thing> tomorrow/next week”.
I’ve been doing this for years. I have it on my phone and computer so it’s easy to view and add new reminders whenever I need. I didn’t think slack is crucial to use but I find it to be the most convenient, even more so then dedicated reminder apps
I’ve heard notion calendar is the best calendar software out there. If it also supports reminders really well (and I’m already a notion user), then maybe this is worth checking out.