Expanding Two-Tier Network

Hey there,
first: I have already set up a few small and simple networks, but I'm not a "professional and trained networking guy" and not sure what would be best practice for the following theoretical scenario:

This is the initial situation - a simple Two-Tier network. The gateway for all clients and servers is Switch A with some ACLs for restricting access from and to guest VLAN.

Let's assume our fictitious company wants to build a second building, 50 m / 328 ft. to the existing one - and this would be the planned situation.

How would you set up the connection between Switch A and Switch B?

My thoughts on this:
1. If Switch B acts as simple L2 switch, traffic between Client C and Client D needs to take an unnecessary long way (L2 Switch -> Switch B -> Switch A -> Switch B -> L2 Switch).

1. What if Switch B acts as L3 switch? Is that even possible? The gateway for Client C and Client D would be Switch B and you need to maintain ACLs on both Switch A and Switch B. What if Client C wants to access Client A (same VLAN, but different gateways)?

2. Am I on the wrong track and there is a completely different approach?

I'm scratching my head and hope you can help me. Thanks in advance!