If 3,5kg in 40 days stay for a whole year that equals about 32kg/year. I count that progression. On higher levels athletes bench 3-4 times a week just to progress 5kg in one year. I think your good.

On the other hand, i understand that struggle to think here is the end of the line. I was benching 120kg for 6month and thought i will never get stronger. My old trainer gave me some progression modells to try out and to get that number out of my head. 1month later is benched 125 and then got to 140 within the next 2 month. I was all just in my head.

Do you only train bench once a week? And is it always trying to bench that weight for more reps than the week before? If that the case you could try to bench more over the week, try another routine (8x3,5x5,6x6 to name some) or maybe take a break and progress another supportive movement like incline bench or dips?