I live half ot the year at 2300 metres altitude, the rest of the year at sea level. When I’m up in the mountains I often ride up to 3000 metres (10 000 feet) and even if I’m acclimatised to 2300 metres it’s much harder up at 3000. I don’t really care to make up a new FTP for every 100 metres of altitude, and I don’t do any intervals that high. I do them below 2600 metres. If I am going for a pr I pace myself with time, feeling, heart rate and power in an unscientific formula. When I race, I race my opponents and not the power meter.
Just like you say, after a while you get an understanding what power you can do at a certain altitude. Nothing a table with numbers will beat you at. 