SUMMARY The Wireguard support in plasma-nm seems to rely heavily on the behavior of the wg-quick tool, and exposes many details that are redundant, given NM's own support for most of these. For example Pre-up, Post-up, Pre-down, Post-down commands are specific to wg-quick and don't integrate with NM (NM has dispatcher scripts for this). MTU overrides what can be set in NM. Most of these fields should be removed, leaving at best "Persistent Keepalive", "Preshared key", and "Listen Port".
The one additional parameter which is not related to the wg-quick tool directly but to the underlying wg device driver itself is FwMark which appears to be unique to the WireGuard interface and doesn't seem to have any corresponding entry method on any of the other screens.
Shouldn't be a problem with Plasma 5.16.