You need to work on something that is easier and you can hold for longer time. 6 seconds is really not enough volume to really start improving. See, you are not even on the chart in the (excellent) article you are referencing, which starts at a max hold of 8s:
With a 6s max hold time, you would have to work at ~70% of that, or 4-5 secs... it is just too short time, even to practice consistently.
The solution is, not to increase the number of sets, but to pick an exercise that you can hold for at least 10-15 secs. Plank with the knees down on the floor, or plank on an inclined surface should work. So, for example, do 6 sets of 8 secs, or 5 sets of 10 secs of kneeling plank. This will give you a consistent base to improve from.
Btw, I think, that the low max hold rows in the table are intended for more advanced stuff like levers and planches. Beginners should work on easier stuff for longer time/more reps, because they can not really effectively/consistently work very close to their limits yet.