All CRMs have their limitations. I think one way to mitigate these limitations is to develop your own modules or apps that interact with the CRM that cover the gaps.

The interaction can happen using REST APIs either on the CRM side or your own app.

For example, if a user enters a new contact in the CRM, I want something to happen that is specific to my company. For this to happen, the CRM calls an endpoint in my app sending the contact's info in the request.
The CRM therefore needs to have the ability to define a url that is used to make the request to it every time a contact is saved.

Also, this should work in the opposite direction. I want to call an endpoint in the CRM to save a new contact. For example, I entered a contact in Excel and Excel can send this contact information to the CRM to be entered automatically by calling an endpoint in the CRM.

I was wondering if there's an open source CRM that supports these scenarios. A CRM that has a lot of API endpoints.

The goal is not to modify the CRM'ss code to add new features but to have it interact with my own app which has the extra features. This way I am in total control of the new features.

If you're unhappy with your CRM and have special requirements, why don't you switch to an open source one and have AI customize it for you? I see people say their CRM doesn't do this or that and they're still spending too much time doing manual work in it.
In this case, customize an open source CRM to address these issues.

Fork the code and maintain it your way.

I know one of the pushbacks is that they don't want to host it themselves or maintain it.
In this case, why don't you hire someone to customize it for you? Let them use AI for that. It will make it a lot cheaper for you and you get the new CRM a lot faster. You test it heavily and if it works to your satisfaction, you tell them to host it somewhere, and you pay them. This is a one time thing.
Then you pay a small hosting monthly or yearly fee to the host. There won't be any per user fees or storage or per X fees. You get all the features for a simple hosting fee and with no limitations.
The hosting company will backup the database for you. Your technical concerns can all be addressed by the hosting type and features. Redundancy, scaling, backups... whatever.

People keep saying you have to maintain it. Once it's running, you don't have to maintain anything.
If an unexpected problem rises, tell the person who did the work to look at it.
If you have a technical person at your company, they can probably fix it.
The person who handed off the new CRM can train you or your technical person.

Every company has its own needs from a CRM, some of which are very specific, and therefore there's no CRM that can do everything you want. Then you will have to create your own CRM or customize an open-source one.