Sure, those setups work (3 routers in very, very large homes). What may not work well or reliably is an AX router configured with 2x AC class routers. Depending on how you set the wireless settings on the RT-AX86U. And my reasoning for not doing so is that you are not getting the benefits of the AX router if you do 'dumb' it down to AC class levels.
A better bet would be to plan to get at least a second RT-AX86U, sooner than later, if needed. A single AX router may (easily) cover the areas now covered by the two AC class routers you now have.
Only actual testing will tell though, in your environment with your clients.
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