To me, RB750 is too old hardware to handle your 500mbps. Even RB750Gr3 also quite hard to handle without fasttrack enable.

Apart from hardware, the diagram is okay. In fact, i am currently using a similar setup. My setup is, i put mikrotik RB4011iGS+RM at the first place (like your storeroom) connecting a Mesh wifi as AP, and i am using a netgear smart switch at the living room. It can do exactly same as your second mikrotik router. You don't need a second router at living room. All the routing traffic should be handled by first powerful mikrotik router. Second area just need a RM180 netgear smart switch to tag and untag the correct vlan and ports. https://www.netgear.com/business/products/s...x#tab-techspecs

DHCP is handled by first mikrotik. You will have same ip range for devices connecting to store room AP and devices connecting to living room AP.

if you put second mikrotik router, you are just putting unnecessary unit to do the second routing work, and you need to have equal power at second router to prevent bottleneck when you are at your living room network branch.

Living room just get a switch that supports vlan will do. Mikrotik RB260GS can do the job.

My house use similar setup, but I didn't route vlan500 over, just 600 and 100(lan vlan)