I’d try increasing the disk queue size, but doubt that would improve a lot. qBittorrent (libtorrent) has deprecated “disk cache” option, so I don’t believe you can set it now. In the earlier days we would increase the disk cache size to a few GBs, depending on the available memory, and disable OS caching policy.
You can also try setting temporary download location to your micro sd, but that would limit your download size to whatever free space you have in your sd card, and also wear it out fast.
Edit: I don’t mean you need to switch to an SSD for the entire storage. You can use a small ssd (128gb) as a tiered caching drive. Torrents will download to this drive initially, and then upon completion will be transferred to the HDD for final storage. You’ll still be limited to 128GB as total active downloads, but that’s a lot imo. The HDD will only be subjected to bulk sequential transfer, and that’s fast.