I notice most replies here are people arguing with you that cloud is better so I thought I would actually give you a potential solution, Essential PIM. :)
It can work entirely off the cloud and offline if you want.
In case anyone else was interested...
In the end I settled on using a traditional calendar program: https://www.essentialpim.com/
Reasons:
Why not something more mainstream like Outlook/Thunderbird/Google Calendar? Essential PIM had some better ergonomic that I need...
I probably sound like a shill or something... na... just pedantic and happy I found something useable. There's also some other minor things in it that I wish were different, and I'm not gunna bother upgrading to the paid version. But it's the best I found for my use case so far. But still keen to hear if anyone know of anything else that might be relevant.
Though not OpenSource I am using EssentialPIM since many years for the same purpose and have taken the Pro license for lifetime upgrades. I think it is a must have software for me. Please do check it out.
I would highly recommend paid version of EssentialPIM. https://www.essentialpim.com/
Have been using since many years. It can sync to it's Android App too so I keep the main version on PC and sync to Mobile whenever I make changes. Do check out the free version for yourself.
Like OneNote, EssentialPim has a hierarchical structure consisting of Notebooks > Sections > Pages. But the app doesn't call them that. Tabs at the top of the phone's screen let you jump to other pages or sections. I haven't found many phone apps that let you do that.
I've been looking into this myself lately and there are currently two software suites in the running for me, maybe you'll get something out of either one of them.
The first is eM Client, it has pretty good free functionality, isn't expensive to buy, has a clean interface, it's used by some major tech companies apparently, and is available for Windows and macOS.
The second is EssentialPIM (Personal Information Management), it has more features and has it's own companion client for Android & iOS, but the desktop version is only available on Windows, and it's more expensive.
Eu utilizo um software para colocar vários tipos de apontamentos pessoais e de trabalho. É o meu preferido até agora apesar de não ser um software exclusivamente com esse objectivo em especifico. EssentialPIM Existe a versão PRO e a versão FREE. A gratuita acho que consegue fazer aquilo que pretendes. O Acess do Microsoft Office também consegue criar bases de dados personalizadas.