Design advice for a 3-building network with redundancy

I've been trying to figure out how I'd design a network for 3 buildings, focusing on high availability.

This is what I have come up with so far: https://imgur.com/a/S1iJT8W

I've got a Layer 2 switch for each floor, and VLANs to separate the wired traffic, wireless, printers, etc. These Layer 2 switches connect to a pair of Layer 3 switches in a dual star topology, which then connect to 2 core routers in the main building. I'm dividing the network into 3 subnets for the buildings, and then dividing further within each building.

The main building is across the road (100m wide) from the other two buildings. I'm assuming for now that I can run multimode 10Gbps fibre under the road easily using existing infrastructure.

There's a few things about this design that I'm not sure about though:

1. Should the DNS/DHCP servers both be in the main building? I'm not sure where they should really go in this design. Is it better to have one DHCP server per building (better in the case the link to the main building becomes unstable), or should I keep it centralised?
2. It seems really expensive to do this design. I'm intentionally prioritising availability over cost, but is there a way to lower the cost without sacrificing much redundancy? I currently have a total of 8 10Gbps fibre links stretching over 100m the main building, which seems like overkill for connecting those two buildings. Is it worthwhile to have 2 Layer 3 switches per building, or could I just replace them with 1 L3 switch or a router?
3. Is there a difference between using Layer 3 switches and routers in this design?

Any help is greatly appreciated!