There is a 3 fold answer on this question.
1_ Take in consideration the length of time used : 3x 7mn= 21mn. This is almost continuous zapping. Your sequence is equivalent to a “standard” zapping done 3 times. Think “elimination”.
2_ As stated by Naomi, following Dr. Clark’s recommendations is a simple and effective way to start with, because 30kHz and 7-20 session gives excellent results (most of the time) and is very gentle on elimination problems. But when this time is over, you have a frequency (30kHz) and a sequence (7-20) as a good reference to try anything else. We are using zappers to experiment. So why stay all the time on traditional tracks?
3_ Many manufacturers offers 2 or 3 frequencies to give you the choice to experiment with.
30kHz fulfills the basic effect of a zapper. You can add the effect of any frequency of your choice and any sequence you could thing of, on top of that (be careful, you are too the “object” of the experience!). Some manufacturers recommends to use their both frequencies (usually 30kHz and 2.5kHz, one after the other), if 30kHz alone is not enough. So, why not give a try to 2, or 3 of them? Have you think of 3 standard sessions of 7-20, each one with a different frequency?
Your idea should not be so foolish. A zapper on the market, the M.Zap3, gives you (almost) your dreamed sequence with 3 frequencies packed in a 28mn_ON, 20mn_OFF, 28mn_ON, 20mn_OFF, 28mn_ON sequence. Frequencies used are 30kHz AND 15Hz together for 21mn, and 5kHz AND 15Hz together for the last 7mn.
I use this sequence quite often as a “maintenance routine” once a week.
If this kind of stuff interest you, check on gooogle for “m.zap3 zapper” or “maestro-zapper”