Greetings,
I worked before with relational (MSSQL, MySQL) databases and document oriented (MongoDb) in SaaS and High volume projects.
We can make a small talk in chat or video messager, probably 1 hour is more then enough. So I can gather some info on your project and provide you with insights I have.
Please answer following questions so I can understand your project better.
The questions that I'm interested in is:
1)How long do you plan to support and add features to your project. How big is it.
2)What kind of queries will be made to the database (how many joins). We can guess it from mockups of UI if you have such.
3)How much data you are interested to store millions, billions, or more in a single table?
4)Planned volume of your application:
Approximatly in number of users.
Is your site public or required authentications (like corporate only)