hard bane
The paragraph here: https://www.home-assistant.io/integrations/thread/#combining-thread-networks says
Combining Thread networks
In the current implementation, having multiple TBRs from different vendors results in separate networks using different credentials. This prevents devices from roaming between the Thread networks. In theory, it would be better to join all Thread networks into a single network to increase the size of the mesh network. A dense mesh network should lead to better RF coverage and better link quality, which lowers transmission latencies, making communication faster.
That describes my problem exactly (my home has multiple competing Thread networks). But unfortunately no steps are listed there to actually resolve the issue. What should I do to solve it?