well i would def. keep up with the vet about getting this done.
free range is nice, but it’s still important for bunnies to have a space they can call their own. not all bunnies can safely be free range, at least not 24/7. many of us have our buns out all day, sometimes restricted to a few bunny-proofed rooms, but they get penned at night.
for a very large breed like that, i’d recommend that you not use a cage at all, those dimensions are not large enough for even a small breed bunny (2×4 feet is the smallest *i think* you should go regardless of bun size).
have you considered an exercise pen? they are 4×4 feet when made square, and that, along with daily exercise will give him plenty of room to move around. another option is the NIC grid condo, but i personally prefer xpens due to the ease at which you can set up and take down to clean. also xpens are not very expensive, but the materials for condos can become expensive once you start making levels…
apple bitters don’t do much in the way of deterring. the smell does not last long enough to make an impact. you can cover hot spots with phone books, cardboard, or litterboxes.