1PG I have watched this thread - it has been interesting. There have been just plain crazy ideas such as approaching removing grease like one would remove soot (small carbon particles) or creosote from a brick chimney. Reminded me of the “theory” of removing virus from blood with a sodium hypochlorite solution.
If you are concerned with whether or not the amount of grease in your system needs to be addressed, there are some steps to consider. Have you read the owner’s manual? How about manuals from other brands of vent hood? Ditto fan.
Vent hood fan systems are not rugged things like the radiator fans we both knew in 1955 Chevy V8s. The blades or squirrel cage are easy to bend. Even a small change can easily cause forces that will wear bearings or result is fan wobble. If that kind of force imbalance happens you are lucky of the fan only makes noise - that is better than the path of damaged bearings. If I really wanted to clean such a fan, I would opt for soaking in solvent, but no physical rubbing, scraping, or spraying with liquids.
The stuff in the vent system is most likely a good bit of vegetable and animal fat that has been partially oxidized and likely become less soluble over the years. There will be other solid materials like dust, dander a smoke from cooking and household activities. If you removed the vent pipe and fan, you could soak it or rinse it in a large tank of solvent. But the solvents that are most likely to work are not exactly absent of fire or health hazards not to speak of smoke and air pollution considerations.
So unless there is a clear and obvious symptom, I say leave it be. Keep the grease filters clean and direct your attention to the stones and rocks in the upper 9 inches of soil in your yard.