Hi Alex,
Once again, thank you for very informative answers!
I think I have soon understood the bare basics 
I have been looking at zigbee sticks - the Zigbee2MQTT page lists some compatible sticks, that also seem to compatible with the other software add-ons: https://www.zigbee2mqtt.io/information/supported_adapters.html
From what I can tell, range-wise the CC2538 + CC2592 - Zigbee dongle, the zzh! CC2652R Multiprotocol RF Stick and the CC2652RB development stick might all be good choices for a long-range setup. The latter two seems to be multi-protocol and not just zigbee. But I am a bit unsure if the CC2592 amplifier would be the most powerful option - or if one of the other sticks would be better?
Also I am a bit confused about flashing - I would prefer a stick that does not need additional hardware to flash, as I guess you will eventually need to flash the hardware to provide support for new devices or updates to the zigbee protocol?
Does anybody have some insight into this?
EDIT: Since these questions about a stick qualify for a new discussion I have made a new thread here: Best most powerful zigbee stick?