Question about Synology RT6600ax

I have a DSL router from Sunrise (older part) and the WLAN is very poor (even on the same floor) and I would like to have a good stable solution for all 3 floors. Sunrise DSL router - goes to a DGS-1210-10MP 8 port D-Link switch and then I want a good router + possibly a WLAN mesh (should be WLAN 6, support MU-MIMO). What should I combine the RT6600ax with for 2-3 separate access points (preferably a mesh)? What is suitable for this or should I go for a semi-professional router (I would prefer to have port mirroring in the WLAN router, but the upstream switch can already do this).