SDF as a base layer would be safer than composite imo (then again SDF is not really a filling but a "bacteriacide". Why not just get ceramic instead of the possibility of other metal alloys like mercury in gold?There are different formulations for the GIC's, but they aren't considered long lasting fillings. Although I think they can take up calcium and fluoride, like dentin, and have some other effects depending on the specific ingredients. The eugenol/zinc paste seems very safe though.
A class I cavity is like a hole in the middle of your molar. There is only downward force being put on this type of filling and it is surrounded by tooth so it's pretty strong. If you move that cavity to the side of the tooth so that it's exposed on the side and the top as well then chewing something hard or tough could cause it to loosen. They have a specific way they drill the tooth out for this type of filling (which I think is a class II) to back it up. Like a 45 degree angle wouldn't be strong, but if you produce a flat spot, or a few steps, it's stronger because the force is directed towards the bottom instead of to the side. With the GIC they like to put an initial filling on the bottom for a class II, then top it off with a GIC so that there is a stronger base (the base would be composite I guess?) There are also resin modified GIC's which are a bit stronger.
When I was researching these things studies made it clear that composite fillings really don't last that long. Like an average of 8 years or something, which I find annoying. And apparently leaking is quite normal so that stuff can get under the filling. It was really irritating when my filling fell out (I think I was eating beef jerky......) although I think I had it for like 12 years so it lasted longer than average. But this is why I want a gold filling, so that they stay in for 25-35 years.