Really depends if on your skills in playing, the best way objectively is just to run skull caverns as iridium nodes (and calico egg nodes if you get mastery around that time) drop a pretty great 50xp per, and can appear en masse as you descend. When 1.6 just dropped I did one massive skull cavern run and got all 100,000 mastery points in a single day.
Otherwise, large scale crops, which will be hastened when you claim a wonderful certain mastery tool that really quickens harvesting. While nerfed compared to other xp (need 2 farming xp per mastery point instead of 1:1 like the other skills), it's still very viable, especially if you have some setup for crops already like on ginger island. 500 starfruit for example (around 3/5 of the island/greenhouse combo) is about 10k mastery xp per harvest, which isn't too bad if you don't need everything right away.
Besides those, you only really have the option of mass reading books which can be incredibly expensive, or slightly less enjoyable alterations of the other two methods like bombing all the rocks in the volcano every day, etc.