I didn’t see anything mentioned in your article about when and how to prune mahonia. I have several that have grown in height but do not bush. And I see that certain mahonia species are are on the bad boy non-native list. I’ll trust the local nursery can help me identify that.
Certified GKH Gardening Expert
Generally little pruning is needed for this evergreen.
You can remove any dead, growing inward or branches that rub others.
After the the Mahonia is finished blooming,
prune any of these undesired branches back to the nearest joint or main branch.
Don't leave large stubs as this can cause disease.
Cut back the plants height or width by cutting the branches to the next joint or terminal.
Selectively select the cuts instead of topping the entire shrub.