Although this is one of the most asked and pondered questions about the post-Civil War era, it is inherently unanswerable. We simply do not know enough to say.
It is true that Lincoln proposed a very lenient plan for letting Southern states back into the Union. It is also true that he vetoed the Wade-Davis Bill. Furthermore, it is true that Lincoln was not particularly racially enlightened. Before the war, he did not call for an end to slavery as the Radical Republicans did. During the war, he did not call for blacks to be able to vote or for land to be taken from the planter class and redistributed among the slaves. Some Radicals called for both of these. This would seem to indicate that Reconstruction under Lincoln would have been much more lenient. Alternatively, the Radicals might have overridden Lincoln just as they overrode President Johnson.
However, Lincoln’s positions described above were all taken during the war. During the war, Lincoln’s one goal was to win. He did everything he did with that goal in mind. We cannot know what he would have tried to do after the war was won. We also cannot know how he would have changed his mind. After all, Lincoln did move to a position where he was willing to issue the Emancipation Proclamation, which is not something he would have done before the war.
Thus, we simply cannot know. Lincoln might have moved towards the Radicals. He might have had enough political clout to make them compromise with him. That could have led to a more moderate Reconstruction that would not have caused so much antagonism in the South. But that is just speculation.