Furmark is brutal, no ifs and or buts. Even has a disclaimer stating its brutal and use at your own risk. There have been gpus damaged by furmark, that's a fact. Take that disclaimer seriously. Furmark is Not an every-day, repeat test kinda software.
That said, the card will choke itself at 83°C, so anything run under that temp is perfectly safe as far as temps go.
Agree with the above. Airflow is nice to have, but not when it's counter productive. Fans work by creating a low pressure area at their front and nature abhors any kind of vacuum, so air will move in the case to fill that void. Too much exhaust up top/rear and you end up with a stream of air that's basically bypassing the gpu. Slow the exhaust fans by half, let the intake static pressure push air to the gpu before it can move up to the exhausts low pressure.
Sys_fans respond to case/motherboard temps, and those rarely reach 50°C, most will bounce around the 40°ish range, they don't respond to cpu or gpu temps which can be double that on the die. So your curves should reflect that range, with intakes staying 30-50% faster than exhausts.
View: https://youtu.be/sh6F2eccMec
Pay close attention to the negative airflow fogbox test, as this is basically what's going on with your pc now, and what you need is a more neutral/positive airflow that'll get more air to the gpu.