You can play with “topology” (subnet) in combination with “client-to-client” options on Openvpn server. Basic routing should be handled already by that foris-openvpn plugin. What usually is problem is to found out what needs to be changed in zones/forwarding/nat/snat …rules. Not every guide written for openwrt or some older TOS release is valid( edit: i mean valid as generic guide …, sometimes there is presumtion you have clear openwrt …or lede or that guide is just part of some bigger guide…)
just-some-notes
I tried several combinations before foris-openvpn-plugin with success and many fails depending how much i crippled the firewall/zones/interface configuration( many rollbacks, “thanks mighty schnapps” 
… since that plugin is in foris i have like two times full reconfiguration of openvpn (second time just to test it after some major TOS update …).
…and my openvpn clients can access all my services in lan (yes i am routing all traffic and using subnet topology and c2c option)…
I have some posts on this forum related to openvpn setup (but for older tos version), but each thread has quite usefull links with generic info …, sometimes i am searching for my own posts just because i can’t find again normally
some-reading-here
Kterak jsem si rozchodil (aspon doufam) openVPN - Všeobecná diskuze [CZ] - Turris forum
ACCEPT vs DNAT (port forwarding) firewall rules - #4 by Maxmilian_Picmaus - SW help - Turris forum
OpenVPN issue TLS server/client - #3 by Maxmilian_Picmaus - SW help - Turris forum
OpenVPN and dns-resolve in LAN - #4 by Maxmilian_Picmaus - SW help - Turris forum
https://community.openvpn.net/openvpn/wiki/Topology
[OpenWrt Wiki] docs:guide-user:services:vpn:openvpn:basic
[OpenWrt Wiki] docs:guide-user:services:vpn:openvpn:basic
https://docs.openvpn.net/connecting/site-to-site-routing-explained-in-detail/