Standard Notes is my go to for notes 100%. It's all about privacy through encryption and is FOSS. Plus for this specific scenario, it also allows for notes export.
Friends used telegram so I had the app installed. It was easier to store documents in telegram than in one drive or proton drive, so telegram became my go too. Obviously not the best setup, but it worked and was easy. I'll try out Signal and Matrix, but continence is a big factor in my choice.