I previously ran a small streams instance on an older 32-bit laptop at home for a couple people. It ran fine. It can also run on shared web-hosting platforms. So I'd certainly say it's lightweight. Though, of course, it all depends on how much usage it will get (number of people, how active they are, how many contacts on other instances, etc). It can use either MySQL or PostgreSQL for db.
As for GoToSocial, you're right, looks like it's intended for no more than a small number of people.
So how many folks are you intending on hosting at this instance?