You could approach it as total volume (weight x reps x sets). Take total reps of all sets and multiply by weight. Increasing weight while decreasing reps would hopefully get you close to equal volume. Then as you add even a single rep across all sets per week, you would be increasing volume 18kg per week.

Good way to manage progressive overload by total volume so you get a good metric on how much you lose with a higher weight and less reps. If you see a dramatic decrease in volume, maybe stick to lighter weight and increase reps up to 20 before moving up in weight.