Keep is an app my wife and i use quite a bit and would love to find an alternative on iOS (if there is also an Android app that would be a huge plus, our son has an Android)
I think Apple Notes may be the change we make. Everything else i am seeing is trying to incorporate google docs and a lot of other things into a sticky note app.
Go for it! It’s not as feature rich as the competitors but once you get the hang of it and have a routine it’s great. There’s also no additional cost. Others oftentimes have a premium subscription tier.
Keep is also a Progressive Web App (PWA) which means you should be able to install it from your browser's menu, without needing a native app and irrespective of your phone's OS (iOS or Android).
Just go to https://keep.Google.com and then check if your browser menu has an "Install" option.