Earlier quoted context omitted.
Got it - I think for the first 1-2 years one broker instance should be plenty. So the decision to up the complexity can probably be postponed to the point where it becomes clear that we hit 10M+/lots of brokers will be required.
Definitely. Focus on solving the business problem first, grow the business. When you have the need to make a switch, you will find a way forward by rebuilding your your MQTT infrastructure to fit your scale. If you ever need to.
Yeah - business is smart home renewable energy stuff, not lightbulbs. If they don't go international real fast the small oldschool broker cluster solution will probably be plenty for the next 5 years.