I’m very sorry you lost your bun. It’s not possible to say what took him without a necropsy. It is however not uncommon for rabbits to pass away unexpectedly. Rabbits hide health problems extremely well, all we can really go by is their appetite, poop, activity level and general appearance. An inner organ problem, for example a heart condition often goes undetected until it is too late.

It doesn’t seem like you did anything wrong, quite the opposite. Free roaming after appropriate bunny proofing of the house is ideal for rabbits. He seems to have been a happy bun -unhappy buns don’t binky. The diet seems perfectly fine.

If you are in Australia you are probably aware that there are rabbit viruses that vaccinations don’t fully protect against. In Europe there are vaccines that give better protection against these diseases (Myxomatosis and RVHD, sometimes called RHD). In the USA and Canada there are currently no vaccines available, but recently there’s been local outbreaks of rabbit hemorrhagic viral disease (RVHD 2) in for example Vancouver and Pennsylvania. Hemorrhagic viral disease is not contagious to humans or other pets, but it kills rabbits very swiftly. You could perhaps contact your shelter and ask if they’ve heard anything about the disease situation in your area.