Einstein was not as good with math but the truth is there are a lot of good mathematicians.
In my opinion Newton’s equations would have been developed by someone else. His logical progressions were incremental and each step in the process was repeatable. I am not saying it would have been accomplished by one person just that is would have been accomplished. I do not believe that Einstein’s theory would have been developed.
The idea that the speed of light is constant and everything else changes. Would not have been accepted unless Einstein was able to predict actual physical results. If he would have been incorrect due to a math error or an incorrect assumption he would have been labeled a quack and dismissed. Even with correct predictions Einstein took a beating.
If Einstein had not been successful, we would have continued to build on Newton’s ideas and just accepted the things we could not predict. Just as we accept gravity but but do know what causes it or how it relates to the other forces in nature.