IDK about sci-fi specifically, but I have scribd and use it extremely heavily, mostly for audiobooks, but they usually have both if they have an audiobook.

Discoverability isn't great, and the catalogue is not comprehensive, but it's reasonably large, especially compared to Kindle Unlimited, and I can often listen to complete series (I mostly read mysteries, personally). Sometimes, if you read several books of a series in a month, it makes you wait until the next month to start a new book in that series or by that author, but I read a lot of books and don't hit that that often. I can give a link to a 60 day trial (you can use PayPal and immediately cancel the renewal through the PayPal interface to avoid a charge) if you want it.

I also use Libby and Hoopla through my library. They have different models and different libraries, but there is a lot you can read for free legally.