(Welcome! I’ve moved this over to projects so you can get individualized responses based on your own set up. We do have some community members who have experience with Insteon, so they may also be able to answer specific questions on differences if any come up.)
As far as core, yes, it’s basically a scripting language for SmartThings. If you have a technical background you should definitely take a look at it.
On the devices, I’m not quite sure what you’re asking. The smart things hub is a single white plastic box that includes a certified Z wave plus controller, a certified zigbee home automation (ZHA) coordinator, and an ethernet connector so it can communicate with the SmartThings cloud. Through the cloud, you could set up integration with pretty much anything that has an open API or and IFTTT service. For example, there is an official Lutron Caseta integration which is cloud to cloud. So smartthings does have one of the widest selection of potential candidates for any device as long as you’re willing to use community – create a custom code to enable it. Different things will work for different people, it just depends on what features you need.
There is also a community – created integration with some Insteon devices if you want to take a look at that in case you can repurpose anything. It’s a fairly complex integration so I don’t think anyone would use it if they didn’t already have Insteon, but if you do it might be worth checking out.
There’s a list of supported protocols in the community – created wiki:
Maybe you can keep the hardware and use CoRE as the go-between. I worked with someone a few months ago using CoRE to make web requests to the ISY controller to control his lights. It’s in here somewhere if you search for it Might be useful reference.