From the M1 Mac mini tech specs on Apple's website:
Simultaneously supports up to two displays:
- One display with up to 6K resolution at 60Hz connected via Thunderbolt and one display with up to 4K resolution at 60Hz connected via HDMI
Thunderbolt 3 digital video output supports
- Native DisplayPort output over USB‑C
- Thunderbolt 2, DVI and VGA output supported using adapters (sold separately)
HDMI display video output
- Support for one display with up to 4K resolution at 60Hz
- DVI output using HDMI to DVI Adapter (sold separately)
I currently use one LG UltraFine 5K via TB3, and an LG 4K (not UltraFine) via HDMI at 60Hz.
I don't think it's possible to drive a second monitor (at any resolution) via a second TB3 port on the M1 Mac mini, as all the bandwidth etc. is used for the 1st port with the active 5K monitor. If you did try to connect a second 5K display or even any type of display in the second TB3 port you'd likely get nothing at all.