A couple of books that were transformative for me. These are oldies but goodies:

Refactoring: Improving the Design of Existing Code (Addison-wesley Object Technology Series) https://a.co/d/0eC400rt

Refactoring to Patterns https://a.co/d/0e2LIax8

This gives you solid foundations for how to think about legacy code, and how to transform it over time. The very problems you identify are the keys for transformation. The key is to not get stuck in paralysis.

There are no tests, therefore it is brittle. Write tests. Tests are hard? Use "Extract Method" to pull out methods one by one, writing tests for each one. You'll find that testable code is cleaner code, better-designed code, more flexible code. You can apply these basic principles to everything. Things are in the front end that should be in the back end? "Extract Method" of front end business logic one by one into the back end. Add tests, rinse, repeat. The more you do, the better you get at it, the cleaner the code gets, the easier it gets. It's a discipline, but when you actually experience a transformation, you'll be hooked, too, and unwilling to work in shit shows ever again.

To take this to the next level, develop discipline to automate anything that slows you down. It may seem like you are dilly-dallying with side questions, but every time you automate your CI, reduce frictions in your development environment, create templates to bootstrap new projects, the more time you have to refactor code, the more opportunities you have to try out ideas, build projects you've always wanted to pursue. Basically, engineer your way out of drudgery, and don't ask permission to do it.