Double check that the Ribbon Tab in your CUIx is configured properly, here's another example:
https://forums.autodesk.com/t5/visual-lisp-autolisp-and-general/custom-ribbon-not-displaying/m-p/754...
One way to easily edit your app's CUIx, is to setup an 'APPS' Profile that has your standard Main CUI, and your app's CUIx added as Enterprise CUI - then I use Robert Bell's 'Edit Enterprise CUI' LISP to programmatically swap them & make changes:
https://forums.augi.com/showthread.php?137223-Enterprise-cui-Main-cui&p=1163984&viewfull=1#post11639...
After making the desired changes to your app's CUIx and quitting AutoCAD, then you need to delete all references to it from here:
%AppData%\Autodesk\C3D 2023\enu\Support
** Change the red to your product or version
Yes, the CUIx you've prepared is located under your app's .bundle folder, but Autoloader copies that to the above path (for each version that loads your app), so any changes you make to the CUIx in your app aren't necessarily reflected until the app is upgraded (Version & UpgradeCode change in PackageContents.xml).
So, the easiest thing to do, is simply make the changes to your app's CUIx, quit AutoCAD, navigate to the folder above, sort by name and delete all .cuix & .mnr files related to your app's CUIx, then re-launch AutoCAD using the desired Profile (not your 'APPS' Profile), and then you should be able to see the changes made.
As for decompiling a DLL assembly... I know a Little Spy. :beer:
Cheers