To address you concern that it is too late once it has been submitted to a conference.
Acceptance: Conference reviews are not the be all and end all. Getting accepted does not make it a good or correct paper. It does not mean that the paper makes a novel contribution. Acceptance reflects the views, knowledge and biases of the reviewers. This type of peer review is useful from the view that it is a a final last check on paper quality, novelty, etcetera. before being put out in the wild to see if your ideas are useful/deepen understanding/correct.
Therefore, you should only rely on the reviewers as a last line of defense against publishing something non-innovative, etcetera. and do a thorough literature search beforehand. You can never be sure, of course. Some authors put "to the best of our knowledge" before making claims of novelty.
Rejection: If you are rejected because it is not seen as novel is not as detrimental as you might think. Firstly, the next time you submit to a conference you might get ignorant but positive reviewers, or the reviewer who thought it was not novel may be someone who thinks everyone is copying them.
Therefore, if it is rejected for not being novel you should not worry too much. Most likely your paper might claim to be the first to do X and Y, but another paper already does X and Y. However, X and Y might be a very broad general contribution. Perhaps the existing work does X and Y using method M1 whilst yours uses M2. Or perhaps existing work makes a more specific contribution than X and Y, X1 and Y1, whilst yours makes X2 and Y2 and both pieces make contributions that could be described as X and Y. In other words, you would need to rephrase how your work is different by making comparisons on the specific details. Now, if the specific details are also the same (never seen this happen, personally), you can at least do a little bit more research to make it different.
To summarise, even if after doing an extensive literature search (which is necessary to make a contribution) you can always adjust how you sell your work (which is sufficient to make a contribution).