How much of my old Feenix folder should be modified to play?

[eholy]

Other than the realmlist and deleting the WDB folder, is there anything else I can do to make sure I am having the best possible experience?

[Landwalruses]

You should be fine so long as it is any 1.12 client, I use the feenix one myself.

[Resolve]

Other than the realmlist and deleting the WDB folder, is there anything else I can do to make sure I am having the best possible experience?

Just edit the realmlist file.