I am using Mail, Calendar, Reminders and Notes as my default apps for my personal and professional work. Below is the link to a thread I started about Reminders.
I’ve tried a variety of different productivity apps but have landed back with the default apps. They are good but like every app I’ve tried they lack features that I’d like to have. The default apps will not work for everyone.
That said, I believe the default apps are more capable than many believe partly because they have not devoted enough time to master them. I am not suggesting that anyone should devote that time; I’m only making an observation based on my sense of things from reading posts in this forum. Do I run into friction, limitations, and frustrations with the default apps? Absolutely! But I have experienced each of those with every third party app I’ve tried. For my needs and workflow I have determined that the default apps have enough features to make me productive while offering the advantages of deep system integration and saving me money. It is a compromise that I’m willing to make.
I use Obsidian for my PKM system as I prefer to keep my research notes in plain text but I find markdown to be cumbersome to use for notes and long-form writing.
I offer the above as my 2 cents, I’m not trying to convince you or others to use the default apps or any other app for that matter. 
Here are few screen shot just to give a sense of my overall organization in Reminders and Notes.
