I'm curious if doing these can spawn them.
Or do we strictly have to do max raid dens in Isle of Armor?
No! It is necessary to conduct Max raid battles in Isle of Armour for collecting Max mushrooms. You collect them only in Isle of Armour and not anywhere else. Max mushrooms are used to brew Max soup. This system (feature) was introduced along with this expansion pass.
Max mushrooms will spawn in these locations. (In Isle of Armour)