I use Nextcloud Notes for that. It has decent mobile app and works great with few notes (Nextcloud Notes doesn't work as second brain very much).

I have two notes in there - Buffer and Shopping list. Buffer is one note for all the small stuff you mentioned. I also have widget on phone homescreen for that note so I can access it pretty seamlessly. Of course, it also means, you need to access some Nextcloud instance (I run my own).

Edit: I use Obsidian synced with Nextcloud for all the "second brain" things.