I had the same issue with slow disks on Hetzner auction servers.
Instead of keeping everything on a single machine, I switched to two Storage VPS from AlphaVPS, each with 4TB.
At around €16/month per VPS, it actually works out cheaper than running an auction server, and the performance (I/O) is much more consistent. I just split my clients between the two VPS to balance the load.
If you go with AlphaVPS, I believe those Storage VPS are now only available in London, but so far it’s been working very well for me.