If you haven't done anything like this before, don't expect it to be easy.
While doubtless many here could cook up an eminently possible project that would take a month of free time learning to even start to comprehend the scope/formulate an approach I have long maintained there is a lot you can do without that. Or if nothing else you can do a fair bit with a tile editor, and maybe even start directing a tracing session in a weekend, and that opens up a lot of potential. Similarly if we have all possible Atari games here then if one somehow is obfuscated and compressed somehow then the option to move onto another arises.
Anyway "better" cheat is somewhat subjective -- infinite lives to bypass the coin-op effect might be exactly what it needed to make a game play for me where the option to turn things blue is less useful, even if it is harder. Infinite items, ammo, health or time is usually quite easy (save for those games where time or something needs to get low to trigger an event, in which case most people try to instead trigger a refill or plus whatever).
The more limited nature of the atari does change some things as you have a little less overhead to play with, but it does also mean some of the more annoying things to work around will likely not be there (if compression is all but unheard of on the NES then chances of it being on an Atari game...). Doesn't need a whole different approach to other devices though as things will still work the same.