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For configuring your openwrt devices you can achieve almost all you want from the LUCI web interface. What you have been reading on the forum is probably related to more complex configurations which is what might be overwhelming you.
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Most reliable in my opinion is copper wire. But this is not always achievable. If you need to connect your devices via wireless then you have different options like you have listed. For now ignore relayd. So your other two options then are WDS or 802.11s (both are considered mesh standards). WDS is the older standard and works across different manufacturers which is why I use WDS at home because I have a mixture of Mediatek and Qualcom/Atheros devices in my home network. WDS is simpler than 802.11s but requires that the wired device acts as a WDS station and the wireless connecting device acts as a WDS client (i.e. it is not peer based). The benefit in terms of openwrt is WDS can be setup straight after flashing your device without installing further packages. This is a real benefit when upgrading to new firmware. On the other hand 802.11s is the newer standard and has more features and more importantly is peer based (all nodes can act has station/client) and will self heal if one of the nodes goes down. The down side is different hardware manufacturers will not talk to each other and also you must install additional packages after flashing new firmware which also makes it less convenient when upgrading to new firmware. Speed wise between WDS and 802.11s there is not much difference. What is most important when setting up a mesh network is the signal between the mesh devices is strong and reliable rather than your end devices i.e. there is absolutely no point that your phone has a strong signal to the remote AP if the mesh wireless connection is weak.
Relayd is used for dissimilar h/w and s/w. So for example I would use Relayd if I was connecting into an access point in an office where I have no admin access to the network but want some kind of pseudo wire.