Right now at this moment reliable connection and FTTP please, haha..
If only 
As you only have a few months before FTTP etc, I totally understand no point in purchasing kit for short term usage.
When you do finally get FTTP, the DSL-AC88U could be replaced by the RT-AX82U, would that cause any issues with Wifi from a central location ?
With current set-up, your Gaming will be laggy as hell, as you say, DSL-AC88U QoS is rubbish and ideally QoS needs to be established from where the DSL-AC88U currently resides , via RT-AX82U or simular.
Note: I am no expert, but I believe once Broadband speeds exceed 400 Mbps, then QoS is no longer needed and Using ASUS AP rather than AiMesh means you have less options available for AP, (Guest wifi etc)
So maybe an option if wifi signal strength is an issue to purchase a second ASUS RT router when FTTP arrives and bin the DSL-AC88U.
Nice diagrams @Chris and @torment 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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