I would like to use opensource PIM apps, and try to replace DejaOffice PIM suite which is an all in one app.
Among many notes/memos apps and after trying many, I found Omni-notes, which has all the functions and is well maintained. (I am still accepting new you suggestions)
However, I have not found yet alternatives for Calendar, tasks… What are you using? is there any interesting app but not in F-droid?