3,500 metres is easy to tolerate. Things get a lot worse. At 6,000m you will struggle to exercise and won't sleep well, even if you're coping relatively well. Above 6,000 m you're pushing your body too much. Above 8,000 m you're approaching the limits of what humanity can survive at rest.
At 3,500 m pressure is about 490 mmHg
At 6,000 m it's 350 mmHg
At 7,500 it's about 285 mmHg
The inspired oxygen partial pressures are 95 mmHg, 63 mmHg and 50 mmHg, give or take a few.
People barely notice 100 mmHg inspired oxygen pressure as being different from normal (150). 63 mmHg is hard work without acclimatisation and unpleasant to live in even for high altitude natives (the highest settlement in the world is just below 6,000m, in the Andes).
50 mmHg will cause everyone problems, and by the time you get to the higher reaches of Everest you will be struggling. For anyone following the calculations, it's not a direct multiplication by 0.21, because water vapour pressure interferes. I got these from Nunn's Applied Respiratory Physiology, rather than going through the calculations myself.
People usually fall unconscious when exposed directly to equivalents of 7,500 m, and frequently do at 6,000m equivalents too. Humans cannot fully acclimatise to altitudes of over 6,000m.
At the peak of Everest, without supplemental oxygen, the work of breathing is so great that even fit mountaineers can barely take in enough oxygen for their muscles and essential functions as they need. Exercise on top of that is extraordinarily tiring. Mental performance is greatly diminished.
It's not an environment you ought to want to put yourself in.
