I have never sailed the new GO 151 so I may not be the best one to ask about this board.
I think you are on the right track, but when I look at the specifications, and what you seem to want, I come to a real impasse.
The GO 151 would seem to be the right board to teach your kids on as it has the center fin.
But it's quite heavy (significantly heavier than your GO 170 I believe).
AND, you are looking for a board that's really easy to jibe (learn to jibe on??).
The "easy to jibe" board, in that size range is the Carve 151, but it does not have the center fin.
I do know that wider is better for early planing, and tail width is also pretty important.
If it were me I would look for another board with a center fin, maybe a Start Small or something,
or get the side fins for your GO 170 and use that as a trainer board.
Sell your Sonic 305, and get a Carve 151 or better still a Carve 141.
Then you would have the best of both worlds.
Early planing and familiarity with the Go 170 (and you can use it as a trainer with the side fins).
Carve 151/141 which should practically jibe itself (I have the 2011 Carve 111 and that's a pretty fair
assessment of the new Carves..... they practically jibe themselves and tend to cruise through carve jibes
Sorry I cannot be more help here, but I've never sailed the GO 151 or the Carve 151 so I'm using the
PR data + the specifications to formulate an opinion that may be correct for your situation, and then again maybe not.
I'm a little puzzled that you have not learned to jibe the GO 170.
Yes, the technique is a bit different as it's harder to set the rail on an 83.5 cm wide board, but if you figure out how far off center to apply your weight, the GO 170 jibes pretty nicely.