This seems way overkill. I understand that that the OP didn't want to pay a subscription, but a really quick google showed that Blink makes a $30 product that solves this exact problem (not being able to hear the doorbell inside), the Blink Chime, which plugs into an outlet and then connects to 2.4Ghz, not requiring a subscription.

I guess $30 for what is effectively a glorified chime for your door is a bit much, but considering the time it probably took to put together this (kind of over-engineered) and the (presumably) monthly cost of the VPS, it makes more sense.

Also, I feel like something like MQTT or another message broker could simplify this significantly, making the whole VPS /state endpoint unnecessary, without exposing your home mini pc to the internet, since the mini pc would connect to the MQTT (I think HA actually has a built in mqtt integration? although you'll still need to make something to process the message once the MQTT gets it) and the SmartThings API would also (assuming the processing after the SmartThings API is hit is customizable in some form)