While I understand the pain- believe me, I'm going through it right now- there is no evidence of different drop rates for different characters. By all indications, drop rate for toons is somewhere in the one in four to one in three range, and it's the same across the board. If you suspect otherwise, collect data. Record about a thousand sims of a node you suspect of having low drop rates. When you're done, you will have a small but statistically significant data set, and probably a seven star veteran.
Changing the drop rate is not an option, because characters do not have different drop rates.
Hero's journey characters are not legendary characters. They're the most special. That seven star unlock is a huge part of what makes it "special," so tiered unlock just is not gonna happen.
Now. I agree that it is very frustrating. For the CLS event, even if I'm going to use precisely zero of the required characters for the arena, most of them are excellent units to have available and strong. Ben, ST Han, Leia, and R2 are all great and useful members of any roster. The only kinda dead weight toon is Farmboy Luke, who's an 8 energy cantina farm who's also available through a hard node and the event shop.
By comparison, two consecutive 16 energy cantina farms for characters with little practical value is deeply frustrating. I agree. And at some point I think a restructuring of character availability is in order.
However, from available options, I'd say the most practical solution would be an overhaul of the guild store and the guild events store.
The other possibility is, with a new table on the horizon with Ships 2.0, that might make certain characters more available, though it's more likely the ship table will focus on ship shards with the odd pilot here or there, so it's more likely to help make Jyn, Gar, and Super Commando more available than the vets.