How to improve the degreasing process?
I don't know why you think bicycle degreaser products are 'weaker for more money' when you have basically proved to yourself that gasoline (petrol) is a very poor cleaner because it evaporates very quickly.
I've had success with a basic citrus degreaser available at Home Depot that costs $4 for a reasonably sized bottle. It's effective at loosening dirty chain lube, minimal brishing of the chain and cassette is required and it rinses off with water taking the dirt with it.
I should point out that you are likely over-cleaning your chain. Too much solvent/degreaser removes the lubricant from deep between the link plates and rollers. I've found I get much better chain life by only wiping down the outside of the chain with minimal degreaser.