Nope. I'm fairly certain he's seeing the same options that I'm seeing. It's 3008 x 1269, then the only other resolution listed above that is 5120 x 2160.
The same display connected to my 16" MacBook Pro shows 3360 x something. That's what both he and I are looking for, and it doesn't seem to be available on the M1.
I have the same setup. I am using the LG 34" with the Apple MacBook Air 16GB/1TB (so same GPU as Macbook Pro).
I purchased it because I was hoping it would replace my 2020 Intel Macbook Pro i7 with eGPU that was needed to drive this display at what I think it the optimum resolution: 3840x1620.
This setting wasn't available by default, even in Big Sur so I previously used SwitchResX to generate a custom resolution. (See attached screenshots).
I have downloaded the current beta of SwitchResX but there is seemingly no way to add custom resolutions at this stage. I have been unsuccessful attempting to disable the Protected Mode so far as I get system errors both in Recovery Mode and booted into Big Sur.
If anyone knows how to insert custom plist files into Big Sur and disable the new SIP then please let me know!
Aside from that, the machine drives the display noticably better than even the eGPU (AMD RX5700XT in Razor Core X) which Big Sur has never liked and punished me with endless system crashes and failures to disconnect.
I think the machine could comfortably drive the display at these higher resolutions and I don't think there is a hardware limitation. I think the default support for 3rd party monitors is probably lacking and hopefully will get better in time.
For those considering this monitor, I cannot recommend it more strongly despite these issues. It is the only display that isn't the XDR 6K which comes close to the internal displays that is this size and colour accuracy.
Let's hope someone figures it out soon!