I've built a dozen or so frames, so am not exactly a pro, but do have some experience. IMO, filling that dent with silver will add quite a bit of strength and will also allow the frame to be powdercoated should the OP decide to go that route. Lower temperature fillers and Bondo won't add much if any strength and also require wet paint, at a significantly higher coast.

Beyond that, I have ridden frames with significantly worse damage than shown by the OP, and never had any problems. This includes a steel cyclocross frame that had been folded up at the HT/DT and TT/HT joints to the point that the front tire touched the bottom bracket shell. It was repaired by pulling it back out under extreme force, and I then "painted" the damaged areas with fingernail polish. I used it for three full cyclocross seasons before giving it away.

So I suspect the OP's frame could be repaired and used with complete confidence.