[Brightcolours]

Why should I set different ISO settings? I am shooting in bright daylight so on both I will use lowest ISO available (100mm in my case)

At ISO 100 10mm f8 on 750D I have more DOF than ISO 100 16mm f8 on 5D will be using same shutter speed will be having same angle or coverage. You might argue about quality pixel density and noise, you are correct, however for me it's the point I noted that matters.

And as said, I am not a wide angles guy, however I discovered this too late after investing in lenses, go figure my prefered landscape lens is 50mm f1.4.

Because you said that exposure time is important. What do you do when exposure time is too long? You raise the ISO.

 

So you choose a FOV. (equivalent focal lengths)

You choose a DOF. (equivalent f-value)

You determine a suitable exposure time. If the exposure time is too long, you set a higher ISO. That is how photography works.  B)