I tried the free model approach for a while, but that doesn't seem to work so well on this site (it worked much better on some other sites). The idea of free models is to drive people to your storefront, but your storefront on this site is so primitive and un-customisable that it's mostly useless. I mean, a customer comes in, sees a single line of 5 of your models (5 more if you have lowpoly and 5 for collections) and that's it. Not even a single page of your stuff.
For every 500 downloads of my free items, I might see 1 sale. And never a thank you or a like. so I deleted them, and it made 0 impact on sales. Still selling at exactly the same rate.
If you're into sci-fi, perhaps cyberpunk might be something. There seem to be a number of cyberpunk games and movies upcoming which has people excited, so getting ahead and having a selection available might work. Just an opinion.
And luxxeon is 100% on the mark. Quality, always quality. Volume is good as you get more exposure and protentional hits, but this site if full of stuff. It's quality that will make sales.
Okay, my 2 cents. I am in no way one of the successful designers on this site, so this is just all personal opinion. Anyway, good luck.