I am now using Asus RT-AX88U, full bar of 5G signal in every corner of my 4 rm HDB flat. As my home getting more and more of Xiaomi smart appliances, so wish to try out Xiaomi AIoT. Too bad, I am on Singtel 1Gbps plan with ONT, can't use China version as main router. Wish to try out Huawei AX3 local set too.
You already have RT-AX88U and I assume it is acting as the main router, in this case, actually you do not want to use Xiaomi AX3600 as the main router, it will be a downgrade from RT-AX88U in terms of wireless spec and FW features.
You can still buy the Xiaomi AX3600 as the secondary router. If your Xiaomi smart appliances are sourced from China, maybe China version of Xiaomi AX3600 will have better compatibility. If your IoT device are from local Xiaomi store, then probably global version will have better compatibility. Set Xiaomi AX3600 as router mode (not AP mode) behind RT-AX88U (Double NAT), I believe your IoT device will still work with Double NAT since they can access the internet.
What is your motivation to try out Huawei AX3 local version (AX3 Quad Core version) other than it is of low cost (S$79 when on promotion)? If you want to buy Huawei IoT device (mostly China version), then you have to buy AX3 China version which is now not worth it because of price increase. Again you do not want to use AX3 Global version as your main router as it is even weaker compared to Xiaomi AX3600 Global version in almost every aspect.