Secondhand Peugeot 408 Engine Failure, Abnormal Fuel Consumption

I purchased my vehicle second-hand when it had only 9,000 kilometers on it. Shortly after, I began experiencing engine failures and noticed abnormal fuel consumption. Initially, I visited the nearest service center and they reset the vehicle, assuring me there was no serious issue. However, the problem persisted just 40 kilometers later. The following day, I had to drive to Çorlu Çetaş, where they diagnosed the issue as accumulated soot in the spark plugs, possibly due to poor-quality fuel. They cleaned the soot and reset the software, after which the vehicle worked fine for about a month in the city. During a holiday trip with my family, the vehicle again malfunctioned, disabling all driving assistance features and experiencing engine failures, traction loss, and erratic fuel consumption. I barely made it to Surtaş in Aydın, where they identified several minor malfunctions which they reset and assured me the vehicle was fine to continue. Upon returning from holiday, the issues recurred, prompting another visit to Surtaş. This time, they diagnosed a faulty first injector but could only reset it temporarily due to time constraints. The vehicle broke down again after about 200 kilometers on the highway, leaving me stranded until I managed to restart it. Frustrated by these recurring problems, I attempted to schedule routine maintenance and address the malfunctions at Çorlu Çetaş, but could only secure an appointment a month later, on July 30th. Given the lack of permanent solutions I've seen reported on social media, I have doubts about finding a lasting fix. I'll update with further developments after my upcoming service appointment.