After a few configuration tweaks and lemmy builds, I was able to increase the upload limit to 100MB and the timeout to 600 seconds without any problems. I kindly ask you to use this limit only when necessary 😅

Also after the Lemmy 0.19 RC update, resource usage increased. So I upgraded the server specs to 16CPU, 32RAM and the thing has improved a bit. I hope these problems will be fixed soon.