I'm pleased that it turned out to be dental problems for your bun, but truely empathise with the financial problems.
My bun also does "spurt eating" &
unusually stops drinking when he needs a dental.
TBH If the teeth don't meet properly (malocclusion) or the jaw joint has stiffened up, we can't stop it happening cos the teeth aren't grinding against each other properly.
There may still be a few bramble leaves left in sheltered places, but not many. It's not so much their mouths. Most buns can handle small thorns. My bun chins his food, the thorns get caught in his fur & work right down to the skin. Neither of us can get them out.
He moults them out eventually in little tufts).
It's possible to rub the thorns off down the central vein starting at the leaf tip. My fingers are tough perhaps a thimble would help? You'll find a great variety in thorniness of brambles & can even buy thornless cultivated brambles & enjoy the fruits in Autumn.
The bark of fruit tree prunings (not stone fruit) & fruit bearing bushes - eg currants / raspberries, is fine too, as is hazel (make sure it isn't Wych Elm by the Spring catkins). I usually use tree/shrub leaves to improve gut motility. Sadly it's made no difference to the frequency of Thumper's dentals because he has malocclusion. Worth a try though.
Good luck. :wave: