Working through Skiena's Algorithm Design Manual  would be a good start and/or watching his lecture series on Youtube .
Other great books include Elements of Programming Interviews  which comes in Java, C++, and Python flavors.
It can help to focus on a smaller subset of Leetcode problems to learn the tricks that underlie most of them. Neetcode  is a good collection of those problems on which you could focus your time and efforts.
I don't think it's realistic to expect to do well on Leetcode without practicing the above skills to some degree, so I wouldn't be hard on yourself if it's been years since you even looked at the material.
Fresh book recommendations delivered straight to your inbox every Thursday.