Hey Ammar,
I'd consider using AerospikeDB. Online mobile game providers like Playtika and Sony Playstation leverage it to keep their users data synched across millions of devices, update leaderboards, provide in app recommendations, player matching etc. Not exactly a notes synching use case but should illustrate the capabilities that are relatable. Another one of the huge benefits is that it will scale both up and out to fit your usage allowing you to maximize the value of your hardware and what's even better... is available directly on the AWS Market Place.
Aerospike on AWS: https://aws.amazon.com/marketplace/seller-profile?id=57c1f592-9dd5-479c-9972-8b27cfc6c230
Download our community edition (It's Free) and better than ever: https://github.com/aerospike/aerospike-server/releases
If your interested in learning more feel free to book some time with me over linkedin: https://www.linkedin.com/in/ldwyatt/