I don't know if this is the answer you're looking for, and you already know this, but socials is a good source for gaining loyal readers/subscribers.
I started an instagram from scratch in March (I think) 2019, and was on it every day either posting content or finding people who were using relevant hashtags and following them, commenting on their posts etc
It was hours every day (early in the morning before work, in my breaks, after work) just on engaging with new people, and maintaining engagement with those who I was already following.
I did my best to make my content as good as I could. I rarely plugged my novel - just because I find it awkward.
Eventually I began getting unsolicited DMs asking about it, and where people could read it. Which was amazing. I get the same asking for commissions, though if you're an artist you're probably used to this it's still novel to me lol
I also put some content up onto pintrest, with links. Got some stuff up on youtube and tiktok, too, but not much.
I still only have about 70 subscribers on my novel here, so I don't know how effective this has been, and I've rarely pushed my novel on my insta followers. But they're some of my most loyal, most supportive readers.
It takes a lot of time, work, and patience (tho probably way less time if you were better at marketing than me lmao)
But you can do this, too. And it can all be for free.
It'll just take time
This was super long, oops. Hopefully it's helpful!
Dunno if you want my insta, but it's here if you want to see - https://www.instagram.com/z.e.falk