I don’t believe heat to be a significant issue with the 14 inch device. I am a fan of the smaller form factor and were I to buy one today, I would select the m3 max chip with the 14 inch form factor, but I am selecting it for the gpu cores. If you don’t plan on needing those, the m3 pro is a good chip.
In a relatively small US city, 600/600 mbps fiber and I actually get it 24/7. I could get 1200, 2400 or even 5000 but I don’t see any point. Heck I can get 700/35 on my iPhone (overnight).