What makes the Yamaha easier to play? More comfortable neck shape? Smaller body size?
You can't change those on the Guild, so keep the Yamaha (at least until you find a more comfortable Guild). But I doubt heavy strings will make the Yamaha sound better. It will just put more stress on the top - as amply illustrated above! I wouldn't use anything beyond medium strings.
But if the Guild's shortcoming is action, neck set, fret problems, or nut and saddle set-up, a luthier should be able to fix it. It's a shame to have to set aside a Guild for a Yamaha!
If they were my instruments, I'd sell the Yamaha to help pay for correcting the Guild.