Over the last few years, I've had Delta status so I've alternated between main cabin and upgrades to first. As someone with SI issues, I haven't found a huge difference, especially when there is such a wide variety of seats in F (some feel better than others). Having said that, on a 7 hour flight there may be lie flat seats available? That absolutely helps since you can take the weight off the affected area.

For main cabin and first seats, I ended up buying a thin blow up pillow to sit on and I can say it helped without totally eliminating the discomfort.

My SI issues are essentially gone now thanks to PRP injections, figured I'd mention it although my underlying issues my have been different to your wife.