Help me to build an office network

Hi guys! Unfortunately, I don't have experience in networks stuff, but now I should build some robust system for my office :)

We have 4 rooms, approx 20 wireless clients and 20 servers. We need to restrict access to servers, only admin can log in/ use network resources (through samba), etc. Our servers should go to the internet using proxies (VPN servers) and we don't want to setup VPN clients on every server, it should be done on the router by static IP rules.

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Now we have only one Linksys EA8500 router with OpenWRT on it. We're using Wireguard VPN and routing is done using vpn-policy-routing package (https://github.com/stangri/openwrt_packages/blob/master/vpn-policy-routing/files/README.md). Anyway, there are no access restrictions for servers. Linksys on OpenWRT sometimes drops connections, restarts interfaces, it's a bit annoying.

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So I'm asking for your advice how to build a robust network for our purposes? What devices do we need? Should we use some server as a router with Kerio or pfsense? Should we place another router between Linksys and servers?

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Thanks and sorry for so fuzzy topic!