I started a game with Purna a few days ago with that exact idea in mind. I thought if I could level up quickly I could get some insanely effective weapons to make my game easier. I was wrong. Very wrong. I am currently at level 50 in the jungle and I am still lucky to get anything above 600 damage. The force attribute has progressed with my level, but certainly not the damage. Even after finding several Orange grade weapons like machetes, double blades, and tonfas my best weapon was a flimsy wakisashi that I randomly found two days ago along the beach. Yesterday I was mid 40's and I was lucky to break 500 damage. For some reason the damage attributes of the weapons just aren't catching up with my Skill Level. I figure that the game increases the attributes of weapons after several play-throughs instead of how fast you level up. It took me a few completed games with Logan before I started seeing any really significant damaging weapons.
Edit: I apologise. I skimmed through your post and didn't read the end. Your strategy is sound but be warned - it's time consuming. One of the fastest ways I have found is by finding an area populated with walkers and any special infected such as Thugs or Floaters and using my fury. Usually during a fury attack your XP will be doubled or tripled based on how many enemies you kill. While I farm for weapons along the beach I kill walkers, build my fury, unleash that fury, and begin all over again. Other than that the only tip I can offer you is collecting alcohol, waters, cans of food, and champagne to collect the XP from the survivors you give them to.