Post-Its on macOS synced via Nextcloud

I am one of the old world old men, who are stick dozens of paper Post-its arround of their monitors edge. But now, I want to transfer these Post-Its into the virtual world.

Do somebody of you know an application for macOS which enables to fade in, fade out notes by shortcut, which are can have beeing synced via Nextcloud?

The icing on the cake would be, if additionally there is an iOS app which allows to create or edit such virtual Post-Its cross-platform.