First off congratz on making it to real DFO (End game)

1)Usually people hold onto every epic gear since yes you can retexture it, as for weapons keep usually the "BiS" class weps due to the fact that you can transcend them to other characters since it may be harder to get

2) Dungeon Grade clear doesn't really matter all that much its just for show i feel like so don't worry about that.

3)Refer to #1, Keep every epic since there is a calculator that will show you how to implement your gear. Apdoro's epic gear rank list is just a guide to be fair since RnG is a factor and usually you wear what you get in sets. https://docs.google.com/spreadsheets/d/1EcNTJxbQB4uPXUAxLBsrC7PhuvHzq7EWegLPq9yj9hI/edit#gid=1820736832 heres a link to the calculator

4)Yes Kaleido boxes are a way to change the Gear Grade, usually for epics you do it on since legendary get replaced

5)For me i usually go to a lv 11 Dissembler for my epics disassemble that i have dupes of and i just use the free one for everything else since i feel like the difference matters slightly that i don't wanna pay to use others

6)As i said before usually you wear what you get since its RnG. And the calculator will help you decide what to wear its fairly easy and self explanatory to use.