Obsidian + syncthing works well for me
Discussion
What is syncthing?
It's a bit of a learning curve to set up, but works really well. I use it between desktop, laptop & phone. I take a photo on the phone and it's on my computers in seconds. Drop a file in a synced folder, boom it's on my phone.
It has limited functionality on iOS though, due to no background processes.
Monitors folders/files and keeps them synced across your devices, has some powerful advanced features too. You can configure it to monitor the folder where obsidian saves your notes on both devices.