If you insist on using a rigid rep scheme you’ll have to increase the weight at some point. To figure out when to do that, something like decrease rest periods, increase rep “quality” or speed, progressively slower eccentrics, etc… until you feel like it’s time to move up.
I know CT has written about “double progression” before where instead of 5x5 you do 5x4-6. Meaning for weight selection if you can get more than 6 reps you increase the weight and less than 4 you decrease the weight. And for weight progression you increase when you can get all 5 sets with 6 reps.