Getting 5120 X 1440 @ 120Hz on a 2018 Mac mini
According to EveryMac, [The 2018 Mac Mini] 'supports "up to" three displays. It can support either (1) two displays up to 4096x2304 at 60 Hz via Thunderbolt 3 and one display up to 4096x2160 at 60 Hz via HDMI 2.0; OR (2) one display up to 5120x2880 at 60 Hz via Thunderbolt 3 and one display up to 4096x2160 at 60 Hz via HDMI 2.0.'
According to Apple themselves, "Support for the following combination of maximum concurrent display setups:
Up to three displays:
Two displays with 4096-by-2304 resolution at 60Hz connected via Thunderbolt 3 plus one display with 4096-by-2160 resolution at 60Hz connected via HDMI 2.0
Up to two displays:
One display with 5120-by-2880 resolution at 60Hz connected via Thunderbolt 3 plus one display with 4096-by-2160 resolution at 60Hz connected via HDMI 2.0 
Thunderbolt 3 digital video output supports Native DisplayPort output over USB-C
Thunderbolt 2, DVI, and VGA output supported using adapters (sold separately)
HDMI 2.0 display video output
Support for one display with 4096-by-2160 resolution at 60Hz
DVI output using HDMI to DVI Adapter (sold separately)