Proxmox, TrueNAS, Debian with cockpit etc. really any type 1 hyperviser work’s.

Maybe your HBA is having issues? or a Drive is Failing? have you done a memtest? you may need to do system wide tests, it can even be a PSU failing or a software Bug.

also TrueNAS is built with Docker they use it heavily something like 106 apps, Debian has good ZFS support, but you will end up doing a lot of unneeded work using Debian unless you keep it simple.

ZFS is by far the best just use TrueNAS, Ubuntu is crap at supporting ZFS, also only set your pool’s VDEV 6-8 wide.

TrueNAS, and Debian

Use a VPN to check for a bottleneck, my ISP will cap my downloads from Steam to 10MB/s with a shitty VPN i get 25+MB/s.