R8 will have advantage for low light vs R7 being an FF camera, I did shoot R8 under summer sun in 4K in a beach and it was fine (more than 2hrs) but it does have temperature guage at slightly more than half way. Interior is fine for R8 as long you got those fan add on, my R8 hasnt shut down on me for overheating yet.
R7 on the other hand is a beast in terms of long form recording, I hardly see the temperature guage showing up unlike the R8 even in direct summer sun, paired with sigma 18-50mm 2.8 you have pretty killer combo (or 17-40 1.8 to give u the FF look)