Transferring a self-written site or how to give a new life to a project?

Question

CMS

Dear experts, your opinion is required.
Given:
-An auto parts website with 1.5+ million products and hundreds of thousands of catalog pages.
-Written in PHP
-No CMS (there is only a desktop program for accounting for the movement of goods)
-500 uniques daily
-1.5 million pages downloaded in Webmaster and 1.1 million in search
-Hosting for 600 rubles VPS 2X2GHz 4Mb RAM copes perfectly
Task:
- Create a Knowledge Base (create and edit pages)
-Make changes to product cards (add photos of goods from the warehouse)
-Work with SEO
-Introduce payment by Cards
-Change the client's personal account
-Change the design
-Add a couple of modules on logistics.
-Add content blocks and manage them
Since there is no CMS, for every change you have to go into the code or hire specialists.
Work is performed irregularly, so the specialists are almost always different. Who wrote what kind of crutches - it's scary to imagine.
Question:
What is the most efficient way to solve these problems? Pull everything on the CMS? What? Are there similar cases?
Forget the idea with CMS and look for a Wishlist artist?
Give a vector of development, I want to humanly make a site for the convenience of users. Eternal crutches are tired. We need a solution, at least for the next 2-3 years.
Thanks to.

Answer 1

80-100t.r. question price.
For how long everything will be saved, you need to look at the site itself

Answer 2

Rewrite to any modern framework: Laravel, Symfony...

Answer 3

Question:

one.

What is the most efficient way to solve these problems? Pull everything on the CMS? What? Are there similar cases?

2.

Forget the idea with CMS and look for a Wishlist artist?

1. You need to design the project architecture based on business requirements.
2. CMS is a small part of business requirements. Everything else is pure coding according to the created TOR, based on clause 1.
I advise you to look here at the plan for creating a technical task and the stages of designing a site.