Glad to hear that you are thinking ahead for your buns and planning on expanding their housing! It’s always nice for rabbits to have plenty of space in their cage so I think your idea of getting a c&c cage is a great idea.
A lot of people use adhesive linoleum flooring (buy it by the roll or square at big box hardware stores) because it cleans up fast and is easy to install and relatively cheap. It can be slippery, though, so you’ll want to have some traction items like straw mats, cardboard, or fleece (as long as they don’t chew it).
Since you already move Benjamin’s accidents to the litter box, I would also recommend using white vinegar to clean up the areas he marked. Sometimes the smell can linger (even if we can’t detect it) and rabbits will continue to have accidents in those areas to mark it. White vinegar breaks down the urine enzymes.
Also, does Benjamin tend to go in the same places or heavier in one spot? You might want to move the litter box to the areas he has accidents the most or get another litter box so he has more opportunities to get it right.
Good luck!