I know the copy and paste works a bit different in 2k21 than tgc2019.In tgc2019 if you used Copy and paste for trees it would place the copied trees at the same height EG if you had buried them a little or made them smaller,In 2k21 when you use the copy and paste it places the trees at the original Height not the copied position....ill take a look later to see how it works with a few trees etc.
It does work in this pic i took 5 trees roughly the same height and placed them separate,Then selected one tree then used the area select to select them all,then copy moved them around a little and then paste then there were 10 trees,then if you do the process again you got 20 trees then again it will be 40 etc...
There is another way as well i used the Y button after selecting the tree and placed it down then lowered the volume to about 0.010 then added more trees to it.....

I think if the trees are close to the fairway usually a better option to plant them separate and change the height and shape etc for better looks above options fine i think if its off the course and fill in the gaps.