While the heat dissipation from the rotor total is based on surface area, the cross sectional area (thinness) has a direct effect on the conduction of the heat from the contact area to the rest of the rotor. This is a non-trivial part of the heat dissipation.

The thinning of the rotor will affect both heat dissipation and strength. Which matters most probably depends on how they are being used.,

My general experience with disc brakes are about 3 sets of pads to a rotor. However, the latest Shimano resin pads are lasting much longer for me. So much so I've yet to replace them after almost 2-years of riding. And I have not had to replace the rotors either. And I do check them. I had to replace the previous version pads about 3X per year and the rotors 1X per year. Though I've been doing a little less climbing lately.

For me, I replace rotors when they reach the 1.5 mm thickness which I can easily measure to several significant digits with a micrometer. Even when that was once per year, I had no issue with it. If I were goign through rotors much quicker than that, I might consider cheating the spec a bit. The OP seems to have found no issue goign down to 1.3 mm. Since the normal wear is 0.5 mm, this is 0.2 mm more or 40% more usage. I'd not push it beyond that.

Also, once beyond the spec, I'd be sure to check the brakes very often to catch an issue at home rather than out on the road.

Good brakes are important and more so with a heavily loaded bike with precious cargo. I'd not skimp on the brake maintenance.