Codenames aren't this magic password that has to be cracked, they're straightforward.
Just like Jack? I explained later, but.
Banjo's codename is Buddy. If you looked at that, you'd instantly know at the very least "This character isn't solo", unless you believe the character was named Buddy.
Using hindsight examples doesn't really help. I wouldn't call that straightforward other than a duo at best. Other duos exist. So again, this is purely hindsight.
Jack and Doyle(Joker and Arsene) aren't difficult to understand at all. A jack is a suite of cards, a Joker is a card. Arsene is the name of a famous fictional Thief, Doyle is the name of Sherlock Holme's author.
Except for the fact nobody figured out how Jack tied to it. Jack could be Jack of All Trades, Suite of Cards, or Jack Frost(less likely the last one). Doyle is closer, but nobody figured it out till it was out. So no, they aren't as straightforward as you're remotely saying.
The only difficult one is Terry, and even then that's just Japanese wordplay like Brave/Yuusha.
I'll always call Brave something not terribly obvious. Again, decent guess we had with a leak slightly supporting it, but not enough details to even get it right. The only time we were right was the Hero Class in particular. But the actual guess was, that was actually treated as the only realistic option "Hero Class, therefore Erdrick". The fact we came to the wrong conclusion makes the "obvious" remark blatantly wrong. If we had come to the right conclusion, it would make more sense.
"テリー・ボガード (Terry Bogard in Japanese)
Terī Bogādo
Bogādo Terī
dorī
Dolly (ドリー)
テリー・ボガード"
Terry nor Banjo's leaked, so that's meaningless at this point.
You can say we couldn't 100% say it was Hero for certain sure, but there was much more supporting it was The Hero than not, unless you were one of those weirdos who took the stance of "WHAT IF IT MEANS THE CHARACTER IS BRAVE?!"
There wasn't really actually anything supporting it beyond a couple of leakers, and the key term is "supporting", not a bunch of leakers taking a guess. Again, it was purely a guess it might relate to it. Brave would've been tied to anyone where Bravery matters. Yuusha was entirely a guess.
So no, definitely not 100% like you're claiming. Considering we did not remotely know why Jack is named that way(there's 3 possibilities. There's zero confirmation of why it's obvious either. Buddies actually is obvious in context. Jack is not with 3 logical meanings to tie him to Joker within the series).
Honestly, almost every name is extremely vague(Packun Flower aside), and is meant to be. Terry is the only one who isn't entirely vague, but even then, people can get that wrong. Geno would've been a logical guess. It was obvious in hindsight that Brave clearly was a name for the Class... and also the name of the character incidentally.
Hero's codename was obvious in hindsight, but was a decent guess beforehand lightly supported by leaks.