The 9mm belts are definitely better and worth doing, but it not like 2x better. If you want to go to screw drive I recommend looking at the Open Builds Workbee. That is designed for screws and would be better than trying to retrofit screws onto an X-Carve IMO. I actually spent less money on my Workbee than I did on my fully upgraded X-Carve and it is easily 2-3x better than the fully upgraded X-Carve. Still not at the level of an all steel machine though.
Get an 800W or 1500W VFD Spindle, not the 2200W. The 2200W it is way too big and heavy. I have the 1500W spindle on the Workbee and it never breaks a sweat. I put an 800W spindle on the X-Carve and it was better than the Dewalt and it was definitely not the limiting factor. If you are doing mostly wood, the 800W is probably enough. If you are cutting AL you may want to step up to the 1500W. The 1500W is hard to find in an air-cooled version. I went with a water cooled version and retrofitted a PC cooler on the machine. It was a lot of work. I’d probably just go with the 800W on the Workbee if I had to do it over.
You could probably sell the X-Carve and buy a Workbee from Aliexpress for not a lot more and have a much better machine. If you just want to tinker and not spend much go for the 9mm belts and skip the screws until you get serious. Definitely skip the z-axis upgrade. I did not see much improvement for the cost. The light x-axis AL rail of the X-Carve is the limiting factor of the design and there is not much you can do about it.