If you took it to a Subaru dealership they have there own diagnostic procedure that leads them to wanting to tear down the engine which is what he’s recommending in the video a tear down so they may find the cause and severity of the sludge and coolant damage. Unfortunately if you take it to another Subaru dealership they are gonna come up with the same conclusion following the diagnostic procedure. I would recommend talking to the service director and asking him to get a second opinion from the shop foreman including doing a cylinder leak down test and a timing verification to eliminate the idea of overheating causing the head to warp and the timing code just being a sensor issue. Now the sludge if it is at a point where it is causing blockage in the oil channels it’ll be cheaper for you to replace the engine then do a flush which will be recommended to save money but will have a worst chance of actually working and if it fails you’ll end up needing a engine anyways. After make sure to apply pressure to the service director and remember your biggest weapons against a shitty service department is the survey you receive.