Best way to set up office network

When our office network was set up there were 4 computers in 2 rooms, so were easily wired to a single network switch.

Now there's about 20 computers in 6 rooms, and the setup now has some computers connected to the initial switch, and some computers in other rooms connected to other switches which are, in turn, connected to the main switch, and then to the router (pic below). Imaging each cluster of PCs to be a separate room. There is quite a lot of internal network traffic as we're running sage networked.

I think it's time to tidy up the network map and run everything directly to a central point. But note that traffic is going to a central (passive) switch and then to the router. Does this create a bottleneck or unnecessary step that perhaps a managed switch would overcome? But I do also need wireless capabilities, and I don't think there's such thing as a wireless managed switch. In which case would I just turn of the DHCP on the wireless router, and that would allow the managed switch to actually handle the traffic?

Any suggestions/comments?

Cheers

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