Manuel Rodriguez Wordpress Engineer (Toptal certified)

If you are short on coding stuff, you need to go probably to themeforest or similar and find a theme that will match your requirements. The downside is, normally, those commercial plugins underperform, but maybe you can beat 90 score with a good hosting and few performance plugins.

I would advise for such a monster DB to keep it external to wordpress. Use elasticsearch for the DB and create an external service/api to be accesible from the WP site.

It really depends, what do you want to manage and who is gonna build the custom theme? Depending on your requirements you can prepare the theme to be more or less flexible and therefore you can get more or less control of it. I'm happy to go into details if you specify what exactly want to achieve.