Currently I have the RT-AC5300 and I was thinking of buying an AX router to setup AiMesh. If I set the AC5300 as the main and the AX connected via AiMesh would I be able to get AX speeds with my AX devices if connected to the AX AiMesh?
Not likely. The AX router should be the main to have AX capabilities within its range.
If you're creating a wired backhaul AiMesh setup, you have a choice to make the AX router an "Access Point (AP) mode / AiMesh Router in AP mode", while still using the RT-AC5300 as the main. Giving you AX capabilities there too (within its range). If this also puts the RT-AX... (RT-AX88U or RT-AX86U highly recommended) in a central location, you may be able to turn off the radios of the main router too and have AX capabilities throughout the home (depending on size, construction type/materials, etc.).
Interesting so if I setup the AX as the main and set the AC in AiMesh mode I will get the AX compatibilities? Also, what are your thoughts on the AX92U?
I can't state my thoughts explicitly on the RT-AX92U here (family forum).
Unless the environment requires many (over ~64) wireless clients, any fake 'tri-band' routers, generally available today, aren't worth my time or my customers.
AiMesh works wonders in wired backhaul settings. Wireless AiMesh is too hit-or-miss for me (mostly 'miss').