Is there some Dropdown Menu functionality in order to select an MQTT broker?

I have read some information about MQTT Dynamic Configurations, where one can force the node to connect to one MQTT or another. I was wondering if this functionality can be triggered by an end-user also using some drop down menus in the flow editor maybe? This would make connecting to multiple brokers a bit more effective as opposed to triggering the inject nodes for connection.

No, because Node-RED has no knowledge of your MQTT brokers nor is there a standard way of discovering them on the network.

However, you could easily build one using Dashboard.