Already know that (alse MagicISO, PowerISO, etc., etc.)...
My preferences go to MagicISO (drag-and-drop, boot tools, etc.) but it is showing some age (2010 last update)
Cant find a good free/open-source tool
Sure, but you have to consider how there is an underlying "futility" in such a tool and that anyway they are about something that has not changed since what, 15 years or so, it's like saying that a FAT12 filesystem driver has not been updated recently.
The CDFS filesystem has been designed as "read only", most of these "iso editing tools" do not actually edit *anything* they often re-write a new .iso in the background, "porting" the current .iso settings.
And historically there were issues, which nowadays, and since a few years, are hopefully solved, still personally I wouldn't trust any of those.
The real issue is that the actual "standards" were not actually followed by everyone or left some "gray" areas "open to interpretation" or that could be read in different ways.
All in all it still makes IMHO much more sense to analyze the existing .iso and recreate via mkisofs or oscdimg, at least for some "less common" .iso's, particularly for bootable ones.