7960p vs 86u -- Same CPU (1.8Ghz dual core), same amount of RAM, but the 7960p has a second 5Ghz radio, so you have the ability to dedicate one 5Ghz radio to bridging/backhaul and the other 5Ghz and 2.4Ghz radios just for clients.
That said, by going non-Asus with the second node, you give up AiMesh and its benefits, such as faster transition roaming and more centralized management. Not sure if you care about those enough at this point, though, but just worth noting.
As far as UniFi Mesh goes, it's meant more as a solution for scaling customer premise access (often outdoors) and is more of a solution to handle growing numbers of PtP backhaul paths, versus maxing out performance per client, as most consumer AIOs are built to do. This is evidenced by the APs themselves being simpler, dual-radio, 2x2 or 3x3 spatial stream, and the firmwares being optimized for ISP/MSP functionality. So while it's a great product, it's likely not the best fit in this scenario, given your goals and environment.
That said, by going non-Asus with the second node, you give up AiMesh and its benefits, such as faster transition roaming and more centralized management. Not sure if you care about those enough at this point, though, but just worth noting.
As far as UniFi Mesh goes, it's meant more as a solution for scaling customer premise access (often outdoors) and is more of a solution to handle growing numbers of PtP backhaul paths, versus maxing out performance per client, as most consumer AIOs are built to do. This is evidenced by the APs themselves being simpler, dual-radio, 2x2 or 3x3 spatial stream, and the firmwares being optimized for ISP/MSP functionality. So while it's a great product, it's likely not the best fit in this scenario, given your goals and environment.