Optimal settings for seeding from a single hdd and read amplification

[sgmihai]

So I am seeding around ~2000 torrents, not very popular, all stored on a single hdd (around ~7TB). My goal is to maximise upload capacity, ideally to fill my 1gbps upload, when possible.
Specs:
cachyos linux, qbittorrent 5.1.2
ext4 with LUKS on hdd
enterprise sata hdd
16gb ram

Only modifications to settings I done was enable space preallocation (does this actually removes fragmentation on hdds?), enable piece suggestion, and 100 upload slots. I seem to be hard stuck around 30MB/s upload speed, with some 40 MB/s peaks, but not sustained. I can easily do gbit upload on that ISP, 24/7, already tested.

I have also noticed some read amplification:

Original image: https://i.imgur.com/wXZ2XiE.png

This is while seeding at around 30MB/s, so about 4x.
That would explain the seeding speed bottleneck, as seeding 50 torrents at once from a single HDD, it can't really go faster than ~120MB/s because of the random seeks.

Any idea how to get rid of this excessive read amplification, and otherwise what could I tune so that I can maximise my upload speed while using a single hdd ? I know ideally i should maybe add another one and do raid0, but I'd first like to know the limits of this setup.
In an ideal scenario I'd seed to the fastest peers only, and only do slow ones when upload capacity is far from maximum. The more seeks the hdd has to make, the slower it will become overall in transfer speed, that is the problems.