If you use a database per customer you should also run a node process per customer. I don't know what exactly you're selling, but that bit of RAM should be in budget alone for the reason that you don't want the requests of customers slowing each other down

I recently jumped over that "trying to minimize resources" hurdle myself because it's just not economically correct for me to spend even an hour of my time when I can host a few months worth of servers for that money