Mounting host folders is already possible using pct set or directly editing the LXC config file after creation. (i.e. mp0: /test,mp=/test)
It would be more straightforward to expose this in the GUI, but there are security concerns...
Most of the cluster requirements are there for a reason. However, you might be interested in PDM [1] if you want a common management plane for multiple PVE instances without clustering them.
[1]...