Plan a Tibet to Everest Base Camp Trip with the less chance of having altitude sickness
The main thing to know about altitude sickness is that it happens to almost everyone, some get serious, and some feel normal. But, if u are frightened about altitude sickness then there are some reducing ideas.
If you want to travel to Tibet then you should go to Tibet through mainland China, not Nepal. As you travel from Nepal, you first land in Kathmandu (1350m) then yo go through the Gyirong port and in about 48 hours, you will ascend much higher quickly which may lead to altitude sickness. So, traveling Tibet through China will have less effect.