I originally got into SmartThings with a ST Link for NVIDIA Shield, and ran into the same exact problem. Long story short, Xiaomi devices won’t stay connected to the Link because of its different firmware from the v2 hub. The issue lays on the side of the Xiaomi devices because they don’t conform 100% to the Zigbee mesh networking protocol.
However, most people seem to have relative success with Xiaomi devices connected to a v2 hub, but there are pitfalls where the use of Zigbee-repeating devices are concerned, as you’ve heard in your original thread asking about this.
I have a v2 hub myself, and the jury is still out on the v3 hub playing well with Xiaomi devices or not, so I will defer to any people on here who have tried it.
However, as mentioned in my other reply, you could also try getting a Xiaomi Gateway Hub for your Xiaomi devices and integrating it with SmartThings using this solution:
I should also mention that Wemo plugs and switches are WiFi-based, so that’s very different from Zigbee-based devices used with a SmartThings hub solution.