Variable speed HVAC w/ compatible thermostat?

My HVAC system is 15 years old. Dual zone, 2 story. The original ducting work is sub-optimal to say the least. So unfortunately I have to replace the system.

I currently have a 2nd and 3rd generation Nest thermostat, one for each zone. The 2nd gen was a carry over from my old house, and I got the 3rd gen when I moved into my current home. This was before I got into home automation. I have the Nest integrated, but I plan to replace them with Z-Wave thermostats.

Looking at a new system, my options are a 2 stage system or a variable stage system. Noise level is a big factor. The 2 stage system apparently will be much quieter than my old system at 74-75dbA. The variable system is much quieter at 55dbA-75dbA depending on speed. And it is more energy efficient. The choice between the two comes down to “I’m spending a crap load of money anyway, might as well spend a bit more for the better unit. It’s a 10-15 year investment”.

My Nest thermostats are compatible with the 2 stage, but not the variable stage. Also my Z-Wave options are very limited. Most being “2 stage cool, 2 stage heat”. The only one that might be compatible is the Trane XL624. The specs are “4 stage cool, 2 stage heat”.

The recommended XL1050 or “less expensive” XL850 (smaller screen) lists on a compatibility table as “Compatible with Communicating and Variable Speed Systems” while the XL624 does not show this. But the XL1050 or XL850 are Z-Wave bridges, and cannot be controlled by a third party bridge. Just WiFi, meaning it needs the internet.

Does anyone know if the XL624 will work fine? Any reason to choose a XL850 instead? Is the only difference just 1 less speed? Anything else?

Is it possible to control the XL1050/XL850 locally without an internet connection? Has anyone reverse engineered them? Any other options?

The XL624 will work theres a native integration. The 824 will as well. For the others therws ways to hack them in at least until Groovy goes away late this year. Do a search in the forums for Nexia thermostat.