Instead of getting an external SSD for torrents, can I get a SSD based USB flash drive with 128 GB or 256 GB (or however big) to download torrents to which I then transfer to my external HDD?
You can but flash drives are not designed for many write cycles. Just use the HDD.
For much the same reason, APFS is likely to be more active managing the storage than HFS+ and so doing more writes to the flash drive.
Going back to a much earlier post:
the torrent starts downloading when added but then slows to a crawl and finally stalls. Then after a few minutes, it starts downloading again but at a slow speed (around 4 MB/sec) and starts fluctuating up and down.
In my experience, typical of torrents. Any TCP/UDP connection is liable to have a saw tooth like speed profile unless managed carefully by the devices at each end - exacerbated by high latency (long distance) connections. Connecting to multiple peers helps to smooth out the fluctuations.
In addition your ISP may actively detect torrent data streams and throttle them. Using a torrent-friendly VPN service can mitigate this.