Which database to choose and how best to organize data storage?

Question

Database design

Tell me which database to choose and how best to organize storage?
I'm making an app for learning words. The database will have a set of approximately 10k words. For each word studied by the user, statistics will be generated - how many times the word was correctly guessed, how many incorrectly, etc. So it turns out that if we say there are 10k users and each has a list of 500 studied words, then we get 5 million records. I think the best option would be to use NoSql and store the list of words in the user's document.
It is also interesting how storage is implemented in todo lists. The principle is the same there, a user can have a lot of all sorts of tasks, which are also grouped according to the user’s personal categories .. But if you store tasks as I wrote above, then as I understand it, it will be problematic to share them with other users, since they will not separate documents. But I do not think that they are stored in a separate collection or in a relational database table.
Generally, prompt as it is better to be in these both cases.

Answer 1

The phrase sounds right

the worst option would be to use NoSql

You have classic ManyToMany

Word:
 name

User:
 ...

UserWord:
 user = FK(User)
 word = FK(Word)
 guess 
 wrong