Did you check that you have same ASIC quality as him ? If he has all cards above let's say 75% ASIC quality and you got unlucky with 70-72% ASIC cards (just a dummy example), then the lower hashrate and the lower settings are perfectly normal. You can check that with GPU-Z.

As for the crashes, maybe a faulty riser, maybe your settings are still too high, did you tune them one by one and checked for errors after more than 5-10 minutes ? It is painful to do, but then you know the good settings for the rest of the life of the rig

Can you maybe help understand how exactly undervolting works under Linux ? Because when you look at Wattman in Windows, things seem pretty simple. You have 7 States and depending on the workload, the driver uses the most appropriate State. For mining, it is State 7 by default. When you launch claymore with cclock, mclock, cvddc or mvddc parameters, you basically override this by setting every state to use those values. When you look at Wattman again, you see all the States have same clock and same voltage, so no matter what the driver decides to do, you are good to go with what you wanted.

In Linux, I find it confusing that you set the State/Powertune/DPM AND the clocks and voltages. If you have your State 3 that is set to 1145Mhz@850mV in BIOS (stored as dynamic voltage in BIOS), asking for powertune 3, clock 1200Mhz and voltage 900mV should be stupid because everything in the State3 is overriden anyway. Why does this parameter appear in every Linux mining distro ? I obviously have missed something.

What is the difference between Powertune/State/DPM 3, 4, 5 when you ask for some specific clocks and memories (1215, 2175, 915mV) ?