What makes industrial capacitive sensors so good?
A few things make capacitive sensors in commercial products like smartphones and induction cookers so performant. It looks like they never fail. It's amazing. Yet when you do it yourself it never seems to be stable.

Commercial appliance and smartphone producers had teams of hundreds of engineers working on this for 20 years on a very few, very specific applications. While you try to do the same alone in a few days.
A smartphone is quite specific. If you use the touch screen of a smartphone outside a smartphone environment it will probably not work better than your DIY experiment. It's performant both because they developed the hardware and an intelligent algorithm specifically for smartphones.
The algorithm is very important because it regulates the sensitivity, the speed reaction and error tolerance according to the user activity. It's many lines of advanced software codes just to detect if a finger swipes the screen intentionally or not.
You won't find the same hardware for custom projects. You can find smartphones parts but then, it will be difficult for you to write the software that works properly with it. At the end of the day you would have re-invented the smartphone.
Appliance manufacturers have chips and hardware specially designed. Chips that you can buy on online electronic stores are multipurpose. They have no idea what you are doing with them. And would be probably very surprised if they did.
EMI and RF shielding is extremely important. You need to shield as best as possible. Maybe you underestimated the importance of it. Shielding the enclosure, the processor etc...
The back side of a touch sensor usually has a trace net used to shield the sensor pads. Full copper areas reduce sensitivity, while too few copper increase the risk of false triggering. This is very subtle.

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