Custom CMS for Every Project

Until now, I have been preparing custom administration for each project again and again, always from scratch. But it is time to create a core that will be reusable, with only the functionality being adapted to each project. Because the core will already be finished, it will save on tokens. In the past, developing a content management system was quite complicated, because when developing a new feature, it was usually appropriate to incorporate it into all the already running content management systems you deployed. It was mostly about security bugs, new features, etc. I am curious where the development of my own content management system will take me and what pitfalls I will encounter.