IMO the simplest fix for Thid would be dropping the cap from 3L0 to somewhere around 2L0 or lower. A group of level 20's pretty much gets wiped out by a temped 2L5+ of a certain class and skills, just an example not meant as a literal fact. That might be fun for that solo person, but the lvl 20's simply leave and I don't blame them.
It might also be a good idea to have gear wear out much, much faster and/or maybe further restrict bonuses from gear while playing in Thid. It only takes a few hours, if that, to make a new lvl 24 and then just swap gear. A new player with junk gear, especially a lvl 20, barely stands a chance against a great number of veteran Thid players on a brand new lvl 24 with transferred gear and fully potted.
Those are my main thoughts on Thid. Many probably don't agree, but wouldn't it be dull if we all thought just alike?