Are you using any of the more advanced feature of the Asus RT-AC86U? It is still very good as a router with a relatively powerful CPU (the CPU is still much better than the CPU used in Asus RT-AX55/56U/58U/82U and Zenwfi XT8). You can actually keep using it and then use the RT-AX55 as a mesh node to see how Wifi 6 improves speed for your Wifi 6 client.
As for the Xiaomi AX9000, local set has the SRP of S$399 and usually the price is above S$300. I believe most of the store will be out of stock for the local set. China set has the SRP of RMB1299 (around S$250) but now the offer price is at RMB999 (around S$200). As long as you are not using Singtel ONT, there should be no issue to use the China set in terms of compatibility. So in the end it depends on your risk profile.
Are you intending to gain SSH access and explore the more advance feature of the Xiaomi AX9000? If not the strong CPU power is basically useless. In this case, if you want to use single router, Xiaomi AX6000 will be good enough and may have a slightly better 5GHz wireless coverage than the triband Xiaomi AX9000. But if you intend to buy a pair for mesh using wireless backhaul, then Xiaomi AX9000 will be much better.
Ref:
https://www.acwifi.net/13907.html (in Chinese)
Redmi AX6000 and Redmi AX5400 Gaming Edition are also good choices for single router configuration.
https://www.acwifi.net/19676.html (in Chinese)
https://www.acwifi.net/18859.html (in Chinese)
BTW, for the articles in Chinese, Google Translate can be used if necessary.