A strong mesh is a better way to go to me - ensuring you have a mesh of router devies between the coordinator and the end device has worked well to ensure that no matter where the device is it works. A better antenna may help but all it takes is a glitch like your 2.4 wifi moving to overlap with the Zigbee range and the device drops out.
I have a tubesb Zigbee device with an external antenna and I’m not sure I’ll benefit from the ZBT2 but the 2.4ghz band is very busy here. I’m tempted to try it and see if it makes any difference. I find my Zigbee network ‘slow’ - like sensor updates take 1-2 seconds before HA receives them.
Not sure what you mean by this - Nabu Casa has a Zwave device already called ZWA-2 which is fully supported.