This community rarely gets off topic. Are oranges named after the color or is the color named after the fruit?
My understanding is that HE arose from ST. As a result a lot of the projects can be ported. There is also a community project called Hub Connect that lets you run multiple hubs - including ST. So you can keep everything connected to ST and start migrating as you are able. I have not used ST myself so I can't really provide any insight there.
If you want to code custom apps then those are done in Groovy. Even without writing custom apps there are already a lot of built in and community apps. Most people find all they need there. Take a look at the compatible devices list and determine if you have anything not on the list. Ask here if you have any devices you are unsure about. Somebody may have a solution that isn't on the official list.
Ask specific questions, start new threads, see what's here. The community has been extremely helpful.