Suggestion for NoSQL database for global users

We are planning to build a social media application, lets say similar to Instagram but at much smaller scale, of course.
The users could be from many geographies like US, Europe, Asia who could interact on same post.
We are evaluating a NoSQL database choice for our use-case where we want:

  • optimal reads at the same time making writes(lets say comments/likes) not noticeably slow.
  • flexible schema, preferably json documents
  • okayish transaction support as it is needed only in few operations and hence we can take a latency hit

We evaluated MongoDB, and with replica set support we can serve reads from local geography(say NA replica for NA users etc) but since there is single master/primary, writes could be slow for some geography(e.g master is in Europe and write came from Australia).
Any help on this would be great.

Note: If you feel this question is broad/needs more info, please comment first before downvoting! I will make sure to add all details that is needed.