Try raptor system's blackbird or a talos workstation.
They tend to provide a good amount of performance to freedom ratio. The only issue you may run into is that they use a different architecture: powerpc. So it may not be the best choice if your looking to play purely proprietary video gayms.
Maybe something from system76 would be better up your ally, but I'm shaken to calling system76 a purely free system. system76 is more like "we tried our best" sort of system.
I hear they ship coreboot/libreboot on most of their hardware (can't confirm) so getting a system from them would definitly be a stepup from building a system yourself.