I have a question on body soldering. 35 years ago I restored a 69 mustang. At the time full quarters were not available and the left rear had been patched several time before I got it. The patches had been pop riveted, welded and brazed. A real challenge for a newbie. I also didn't have the very real benefit of You Tube. I used Eastwood body solder and a lot of their other products, which were the only source of info besides old timers advise. I ground out all the crud and soldered the seams and after 35 years they actually held up well. I was impressed. The Deltron paint was perfect except for where the body filler popped off. The car has always been in Colorado so it's seen some temperature changes. -15 to 110 F. I could write a book on what I've learned from taking the same car apart 35 years later! This time around I have new full quarters to replace the patched panels and my question is this. Is there a better/newer option other than to use body solder for the sail panel joint? The factory joint had about 3/16" of solder in the seam. I enjoyed the challenge of soldering since most of the body guys back then didn't do it. Too hard they said. Once I figured out that it had to be shinny clean it wasn't hard at all. So, is there a filler that is flexible or has the same expansion rate as steel that will not fail? The joint is mechanically solid since it's spot welded to three different panels but the thermal expansion will be the problem. The body filler I used back then is THE reason I'm doing this again. It was PPG but it was 35 years ago. I'd like to think it has improved.

I still use body solder for those deep factory joints. I wouldn't trust filler that thick. I'm sure others may use Gorilla Hair or other fiber reinforced fillers, but I prefer to use what the factory did, and I know works well and lasts almost forever.
They do make a lead free solder now that's safer to use ( not that lead is very unsafe). It takes a slightly higher temperature to melt, but I think the working window is larger. I've used this on parts to be plated, as it is harder, but not on panels yet.

I have seen some who cut out the seam and weld a patch in.
It sounds like you were successful with your previous lead work, but if you decide to use it again make sure you don't blast it or sand it with aluminum oxide sand paper, it will cause corrosion. You can sand it, but use carbide or ceramic abrasives.
Here is an example of the corrosion---you can  see how it has raised the sand scratches. It's a 1/4 panel top seam that was done just over 30 years ago.

30 years in the business. Never had a failure using short strand followed by a quality filler. My daily driven 67 Camaro was done over 15 years ago and the sails are perfect. I fully weld sail and A-pillar seams prior to filler. Countless jobs I've done and seen throughout the years done this way with zero failures when done correctly.
If your bodywork and paint fails it is because you screwed up the job and failed to gain correct knowledge.