IsItCool - you don't need to look too far away for ideas - take a pleasant drive to Sequim and compare their soil and sunshine to yours; they are known for their lavender. Theirs is less clay, more sand and flint...more sun (well, in a normal year). I also think the landscape cloth was not helpful. I think that you did no favors with the addition of compost and OM and might have been better served with grit to aid drainage. In spite of that, really your plants look pretty good.
Something to keep in mind - when discussing plant longevity some of that relates to the plant producing the best essential oil and unless you are extracting oil plants can go on fr quite a long time - judicious pruning while plants are still fairly young helps prevent the woody dead-interior look. I also prefer angustifolia, I can generally only find "Hidcote' and now El Pollo really makes me want to find 'Havanna Dark' and 'Forever Blue'. If I remember correctly 'Phenomenal' was bred for areas with high humidity (I remember that it was being introduced when I was living in Memphis as a lavender bred for the Southern climate).
As you say - some lavenders seem to do just fine for many years with very little attention - but they are growing where they want to be in the well drained, low nutrient, hotter and sunnier micro-climate and having been growing there for years without attention their soils have likely depleted to their liking. Sorry but those general conditions are just not what comes to mind as I picture getting off the ferry in Clinton.
