OK, I got some advice from my local TTN community. For anyone else that might be interested, and to put and end to this thread, the advice I got, which seems perfectly reasonable is:
- Create a new application on the v3 stack.
- Create an incoming MQTT endpoint wired to the v3 stack for data ingestion into your system.
- Start creating new devices on the v3 stack.
- Deal with migrating v2 devices to v3 later (this summer).
- Deal with migrating gateways sometime after that.
Other notes:
- In v3, App EUIs are essentially arbitrary and you should just set them to 1 (not the default of 0) when you create new devices in V3.