You might find this comparison interesting and useful if you have doubts about what solution would be appropriate for needs of your business - customized CRM or off-the-shelf solution. Each solution, customized CRM and ready-made CRM, has its pros and cons - here's a quick run down.

https://joinsoft.com/blog/an-ultimate-comparison-of-custom-vs-off-the-shelf-crm-software-solutions/

Possibly, this article may be useful if you have question how to create CRM software.
https://joinsoft.medium.com/reasons-to-create-a-custom-crm-system-for-your-business-635f28c8e6e8