As you can see, the last 6 digits correspond on both
Correct! The Code Breaker code system is actually a mini-language. The first two digits indicate a code type. Many of the code types use the next six digits to indicate the address in RAM that will be changed and use the second line to indicate the value to put into that address.
As an alternative to the verbose GameFAQs article I linked to that explained it, here's a shorter description of the code types:gamehacking.org faqs - Sega Dreamcast Code Types
I was able to play around with it and got the following code to work: see update belowSonic Adventure v1.1
(This code means "If address 8C760970 contains value 0001, then set address 8C760970 to value 0002.")
Remember, as the TCRF page says, for this US version 1.1, the level list appears as all blanks, but you can still move the cursor to select a level and use X and B to select a character.tcrf.net: Sonic Adventure - Level Selecttcrf.net: Sonic Adventure/Version Differences - Menu Differences
(scroll down a bit to see the different Level Select screenshots)
While the level select code above works, it only lets you access the level select menu one time after the Sonic Adventure disc starts up. If you want to get back to the level select menu, you have to start up via the Code Breaker disc again.
Here is an updated code that will replace the title screen with the level select menu. This allows you to pause and quit to get back to the level select menu. (Note there are some combinations of levels and characters that don't quit back to the menu or will eventually crash.)Sonic Adventure v1.1
00000002(This code means: "If the menu ID is the title screen [address 8C81E904 contains value 6], then go to the level select menu [set address 8C760970 to value 0002].")
Here is a code that will let you see the Tokyo Game Show version of the menus:tcrf.net: Sonic Adventure - Tokyo Game Show LeftoversSonic Adventure v1.1
Tokyo Game Show menus
0000000F(This code means: "If the menu ID is the title screen [address 8C81E904 contains value 6], then set the menu ID to the Tokyo Game Show version of the title screen [set address 8C81E904 to decimal value 15].")