@korayberk I am replying here below in this existing hread instead of in your new thread as you asked not to be lectured there and this might feel like a lecture → Is Home Assistant Yellow really unusable?
Choose to ignore this if you want but I am telling you that this will explain your problem and the solution.
Firstly, I believe that you have misunderstood what “zigbee network utilization” values mean here. That “zigbee network utilization” has nothing to do with the amount of Zigbee devices on your Zigbee network, so it has nothing to do with how many more Zigbee devices you can add. You see that “zigbee network utilization” is just an the the result of a frequency scan that is meant to be used indication of how much radio noise is on those frequency bands which is meant to help you select the the Zigbee channel with the least radio noise. Most of that radio noise will be EMI (electromagnetic interference, also called RMI / radio-frequency interference) from different EMF (electromagnetic fields) due to all kinds of electric charges, meaning everything with electricity, including all types of cables and electric appliances.
You can use that “zigbee network utilization” for troubleshooting to do tests as you move your Zigbee Coordinator away from sources of radio interference, usually starting by moving it away from any devices with Wi-Fi router/access, but also all other electronics. FYI, other than WiFi, know that USB 3.x/4.0 devices (like example USB devices such as harddrives) are infamous for causing serious interference with Zigbee communication. It is therefore highly recommended that you read and understand then follow all these tips → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
It is not unusable, you just need to make sure you are not using the built-in Zigbee radio as a dedicated Zigbee Coordinator and not in the experimental RCP Multi-PAN/Multi-protocol mode (please see the detailed explanation of why that is further down), and you also need to understand how to work around the inherent limitations and network design of the Zigbee technology.
The radio module built into the Home Assistant Yellow is based on Silicon Labs EFR32MG21 SoC and that has been proven to run stable when handling around 200 Zigbee 3.0 devices if running as a dedicated Zigbee Coordinator in NCP mode for a single network and as long as you also are following Zigbee best practices. (But if you have enabled the experimental RCP Multi-PAN/Multi-protocol mode then that goes out the window, again please see the detailed explanation of why that is further down).
That however depends on you also adding enough Zigbee Router devices (i.e. mains-powered devices that work as good Zigbee signal repeaters / Zigbee range extenders), as well as do your best to keep sources of EMF/EMI/RMI away from all Zigbee radios. That is, you have to understand that Zigbee heavily depends on its Zigbee network mesh technology which depends on having many stable Zigbee Router devices on the same Zigbee network. Again, I highly recommend that you read and follow all the tips here → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
Note that all the above is only true if you run it using the default Zigbee-only mode (with the standard NCP firmware) and you have not enabled RCP Multi-PAN/Multi-protocol mode to also use it for a Thread network. Please see the detailed explanation below of why that is and why you should not use RCP Multi-PAN/Multi-protocol mode if you want to use more than a few Zigbee devices.
Just to clarify my recommendation about running in NCP mode with a single private network instead of running in RCP Multi-PAN/Multi-protocol mode with two active private networks on a single radio (one radio SoC/MCU module);
If you have enabled RCP Multi-PAN/Multi-protocol mode support on the built-in Silicon Labs EFR32MG21 radio module in the Home Assistant Yellow (and thus switched that single radio to be used in the experimental RCP / Radio Co-Processor mode) then connected both Zigbee devices and Thread (Matter over Thread) devices to that single radio at the same time you can not expect it to concurrently be able to handle as many total devices as it would be able to handle if it was instead being used as dedicated Zigbee Coordinator in single network mode.
Because of the way Multi-PAN/Multi-protocol technology works, running in two networks with two protocols simultaneously will significantly limit the total amount of devices that a single radio can handle, so just because that single EFR32MG21 radio SoC can handle around 200 Zigbee 3.0 devices when running as a dedicated Zigbee Coordinator in NCP mode does not mean that same single radio can handle 100 Zigbee devices and 100 Thread devices concurrently. Also understand that the way Zigbee works the resource constraints of that single radio is a bottleneck for the whole Zigbee network.
The fact is that the overhead and complexity of running two different networks in RCP Multi-PAN/Multi-protocol mode will realistically restrict the total amount of devices that the single radio can handle. Hence the RCP Multi-PAN/Multi-protocol mode is still being labeled as experimental, and the general recommendation is to instead use a separate dedicated radio adapter for Zigbee once you start adding many Zigbee devices.
So do not get me wrong, the built-in radio in Home Assistant Yellow is great to get started with both Zigbee and Thread, but once you added 50+ devices then I would strongly recommend that you separate those two networks so that they run on their own dedicated radio adapter/module.
Finally, to keep things fair you should start by comparing apples and apples. To compare, the Philips Hue Bridge officially only supports connecting up to a maximum of 50 Zigbee devices, and that upper maximum is similar to most other commercial Zigbee gateway/hub/bridge/controller appliances, so compared to having multiple Zigbee gateways/hubs/bridges/controllers it will be much better to migrate all your Zigbee devices to a single network, read my related comment here → Zigbee buyer's guide - #3 by Hedda