Modify an Existing Website

I have an existing website. I need modifications in the same.
1. Code Restructuring(More Modular Codebase)
2. Performace Improvement
3. UI Design Modification(CSS, Responsive)
4. Addition of few functionalities
5. Bug Fix
6. Full Testing

Tech Stack: Angular 8, Firebase, Python

Note Only Experts required. I will be very much focused on quality primarily.