14 days after the 12/12 transfer, should I do lolipoping or wait until the 21st day?

You can pretty much do it whenever. The day 21 rule is more so about doing it before day 21. Some people do it right before the 12/12 flip Some do it at the flip and then again at day 21 after the stretch. I think the idea behind waiting until day 21, is that the stretch period will be over around day 21 and you'll be able to see which branches aren't going to make it to the top of the canopy and you can get rid of them. Sometimes certain lowers will stretch right to the top