Assuming you get a Mavic 3 Multispectral here is a table comparing mapping speeds and ground sample resolutions at different flight heights. It also makes a difference if you fly only RGB or RGB+MSP as the Multispectral sensor is slower:
Based on this data you will see, that mapping at 12m height is unfeasible for practical field sizes like you mentioned. Atleast thats the case for the mentioned DJI M3M , if you for example have a 60MP sensor on a another drone it looks completly different, as you can fly higher and cover more hectares while maintaining good ground resolution.
Now some practical adivice:
For Green-on-Brown targtes (assuming every green plant on your field is a target and the rest is soil or fallow) I would fly between 70m and 110m and use the multispectral imagery. Because with that you can create very sensitive vegetation indices lile the NDVI. Even though you will not be able to ID the weeds or volunteers , you will know where they are! With PIX4Dfields you can then create a map which selects only parts where “green” pixels are.
@ 70m you would be able to map 100 hectares in 2h with multispectral
@ 90m you would be able to map 100 hectares in 1,3h with multispectral
@ 110m you would be able to map 100 hectares in 0,8h with multispectral
If you only fly in RGB which also works, but then you are limited to VARI and TGI indices which are a bit less sensitive (so you might miss some small areas) you could already map @ 90m with full speed of 100 hectares in 0.5h.
Here you can see how such a workflow looks like in PIX4Dfields, and that was based on RGB imagery only:
