Free is hard.
Aside from newsletter swaps, which can be difficult to arrange if you're not in social contact with others who have books very like yours, there are a few options.
There's a great facebook group for SF/F swaps here:
https://www.facebook.com/groups/1163390753699773/I use that almost exclusively. Very welcoming and helpful people, and even when I had a list of 200 I don't recall anyone every knocking my offer back. (Well, not for that reason. Sometimes I offered a freebie, and some folk don't promote free books to their ML, which is fair enough.)
My list is pushing 1900 now, just 5 months later, and I'm working hard on signing people up organically. I use perma-free short stories (wide) with irrestable free book offers inside.
I rarely do swaps now, but only because now I have all my first-in-series set to $2.99 instead of free or 99c. My strategy is to apply for bookbubs with each book every 30 days, and I can't do that if I drop the price. On the other hand, if I get a bookbub I'll try and find some swaps for when the price is reduced.
By the way, Bookbub CPC ads are amongst my favourite tools. I'm running two at the moment, one of which is doing well, the other not so well.
Both ads are for books at $2.99, wide, across four countries. It's great to see 2 clicks on the google link for 75c each, then 2 sales from Google the next day at $5.50 royalty. And, hopefully, four more sales for $14 if they buy the rest of the series later!
I was thinking of asking Tim about a sub-forum where we could share and dissect bookbub ads. I'd love some feedback on one of mine, but sharing an ad is a bit like ... advertising.