There would be no less notes(ie reeds) if you took out the dim and seventh bass rows. Those chords are all made by combinations of the 12 notes. The bass section has five/4 sets of one octave 12 notes double reeded plates. You would save the button weight and the weight of the rods that's it.
I have a German two reed 80 bass (5 rows x 16, no diminshed notes)and it weighs only about 12 lbs. I also play a Baffetti three row. the three row bass side is very useful and tho it is only has the trad 12 bass, it covers most of my objectives. That said, the piano accordions can be bot as 12, 24, 48, 60 bass. But, in order to get the precious E, Em, or B notes you have to get +48. contact accordionheaven.com or baldoni.com and see if they have some odd-ball configurations. I would be interested in seeing a 3-reed 34 key piano accordion with a 24 bass that has an E chord, both major and minor. For those of you who seeking orientation and comparison to a cajun 1-row: a 12 bass piano accordion has these 6 root notes (from low to hi): Bb,F,C,G,D,A and their counter bass chord, making a total of 12 buttons. However, I have never seen a three-reed 12 bass. But I am sure they are some where. or maybe you can build one with a three row left hand and a piano right hand. I believe the Russian 6-row accomplishes this as much in its chromatic configuration but, no keyboard. Sounds like total reinvention so, maybe a solution for the programmable FR-18 and see if your idea is workable.