That''s the exact same cycle that we went through.
(The previous owners were not as careful keeping oil/diesel up out of the bilge).
Simple green or the orange cleaner (both marked bio-degradable at home depot) made a big improvement.
Then I tried the small 120v ionic breeze bathroom-sized model tucked down into the engine compartmet for a week. It made enough of a difference that we spent the $$ for the bilgebuster.
Never regretted spending the money ..... You may want to call the mfg direct and see if they have any specials/sales going. Ask if you can trade-in the car ionizer as a trade up to Bilgebuster. I found them very willing to work with us on buying direct, favorable to our electrician''s wholesale quote.