Hello!

In the wikidata key the the wikidata id of the monument itself should be added, e.g. here https://www.openstreetmap.org/node/2315294665 both the statue AND the subject of the statue has a separate wikidata pages. Just because a memorial doesn't have a wikidata page (yet), it does not mean we should reuse that tag for a different purpose.

https://wiki.openstreetmap.org/wiki/Key:subject:wikidata

It's called "subject" because a memorial is not necessarily commemorate a person, it can be anything else.

Thanks! Have a good mapping!