If you push on the heels bulbs with your finger, are they gushy or does your finger bounce back? The more "bouncy" they are is healthier.
I would hope that regardless of the health of the foot, heel bulbs would 'bounce back' when pushed on! The alternative would be alarming! I imagine it's just the way you put it Puck, so not intending to contradict, more to clarify...
The back of the foot(frog, digital cushion, lateral cartilages, heel bulbs) should be large, thick & firm enough not to yield much at all to strong thumb pressure. Eg. Put your fingers on the frog & thumb above the digital cushion( in the indent above heel bulbs) in front of the frog stay(where the central sulcus 'dent' is) and squeeze. Is it thick & firm, or does it yield easily & feels only an inch or so thick? Palpating the lateral cartilages should find them firm & thick too, not only a 1/4" & easily moveable.
Unfortunately IME the opposite is the common state of affairs and by the look of that foot, it may not be too bad, but betting it's not that strong, thick & full of fibrocartilage either. Therefore I'd be wanting to treat for thrush & then consider it may be necessary to protect/support the frog(& what's above) with pads before working the horse on hard ground. All very well saying they need use to grow strong, but if they're currently too weak to allow the horse *comfortable* use, he'll be 'tippy toeing' to avoid them.