Regular 4k@120hz is totally doable on M1 Max MBP, can confirm that myself.
Hard to answer the question regarding 4k+ ultrawides @120hz though, primarily because i dont think there is a display like that in existence yet (googling didn't net much success there for me). The only ultrawide 4k+ display i found that is above 60hz is the LG 5k ultrawide with thunderbolt 4 support, and it maxes out at 70hz (and M1 Max MBP supports it at that refresh rate just fine, that's my current setup).