It's been the same for me. 2 years ago, when I discovered Lemmy, the platform felt a bit dead. I looked around and there wasn't much content.
Now with the huge influx of people, Lemmy has seen an unprecedented grow. It's undoubtedly caused it's fair share of problems for the communities that were already in existance, although I don't see that as somerhing bad, just a step in it's growing process.
Way before the meltdown of Reddit, Lemmy was used mainly by it's creators and likeminded people. Now it's become way easier to find interesting content and diverse opinions.
I hope this wave of reddit refugees has given Lemmy the exposure needed to become a useful platform. I just hope that "the bad" of reddit doesn't come here.
You could try Sci-Hub and LibGen. If you don't find what you're looking for, you can always email the authors or other scholars who may have access to the paper you're interested in. A less alternative site would be arXiv, but it just has preprints.