Hi.
I am Pranav.
I just read through your project description in detail. (Thanks for mentioning all the relevant details in the description, many people overlook it).
My first question to you would be if it is absolutely necessary to use the doorbell circuit. The same can be made with Wifi or BLE as well, or at sub-ghz proprietary RF. If you could tell me what the receiving side device would be like, I could help you in selecting the most profitable technology for you, and make a circuit design and PCB for the same.
I can also help you with prototypes through my contacts in China.
There are a lot of possibilities that are coming into my mind regarding this project, but obviously I need to know the entire system to suggest the right option. You can share the details under an NDA if required.
As for me, I am an electronics hardware designer nd have actively worked on several projects consisting of various wireless technologies. These include BLE, WiFi and sub GHz RF. I have also worked on power control devices and high power electronics, along with many other general projects.
I am a sole designer here working on my own along with links in China to help with component acquisition and prototype manufacturing.
Hoping to read from you.
Regards,
Pranav