Clogging Drain?

I have a kichen waste pipe that caters for a dishwasher outlet and a double sink. A 40mm pipe runs in the wall and underfloor for about 6 metres and then exits the building. At this point there is a fitting that joins the 40mm to a 50mm pipe. About 1 meter further is a 90 deg bend with an inspection hole. Pipe then runs another 8 meters to two 45 deg bends which also have a inspection point between them and then into an underground 200 lit grease trap. Despite being very careful not to put grease down the sinks the system clogs up regularlywith congealed fat in the pipe just before the first 90 deg bend. The reduction fitting 50 / 40 mm also makes it hard to shove a hose up the pipe to clear it. 40mm is a bit small and the fall is probably not great either. Was thinking of doing the following. 1. replace the reduction fitting with a better one. 2. replace the 90 deg bend with 2 x 45. 3. put a vertical vent pipe in the last section just before the grease trap as there is no other vent and we sometimes get grease trap odour in the sinks. The idea being that this may help to prevent vacuum forming which slows flow and allows grease to accumulate. Would 30 cm or so be tall enough for a vent pipe? Thank you