I just did a random search on Ali for "T12 Handle", and among others, the link below popped up.
https://www.aliexpress.com/item/32965564853.htmlIt has some pictures of the inside of various handles.
Some have 2 pieces of FR4 wit single wipe contacts, others have a plastic tube with moulded in connectors.
About the "fit" of the handles with KGSER station. It's just a connector on the end of a wire.
The only extra's in the handle are a tilt switch to detect movement, and an NTC, and I think this is to detect / prevent overheating of the handle itself. Apparently both 4pin and 5 pin connectors are used, so you may have to pay some attention to that, or just buy some extra connectors.
You can also buy the wire between the handle and the connector separately, or bundled with a handle as a set. The cheaper versions probably are PVC, while the better versions have silly cone.
You can also buy the control PCB with O-LED and rotary button as a spare part for just over EUR20. A spare may ease your mind if you have intentions of re-flashing the software to remove some "feature" that irritates you. I think a bunch of firmware variants float around on sourececode distribution sites, packed in git repositories.