When using the SmartLife - ST integration it will let you choose what SmartLife app you want to integrate and Nedis will possibly be a choice. You will need to try it and see what options you get.
Many or all of your existing ZigBee devices should be able to pair with your ST v3 hub. In many cases you will need to find and install community written edge drivers.
The ST v3 hub works with Matter over WiFi, Matter over Thread, and Matter over Bridge.
Aqara ZigBee Devices can be directly added to ST. With a good ZigBee mesh they should work. You could also get an Aqara hub that is a Matter Bridge and use it as the connection.
Tuya (Denver) based cameras will likely pair with the SmartLife app but will NOT show up in ST.
Between Routines, API Rules and SharpTools you you be able to replace WebCore.
NodeRed is an option but it would require you to run a sever to use it.
API Rules still requires coding but there some threads with examples you can copy and paste into the Rules API using the Advanced Web App (AWA). Here is an example.