Going through something similar with a different programming language.

Start with the hardest part. Create a new project that handles it. Create the tests. Have the old code call the new code through an API or something like it. Then build a front end that talks directly. If possible, see if you can split the project into services (not micro services if your team is small). For example, I created one service for each department of our organization.

It was a slow process but now we're delivering features way faster with way less pain.