Rare metals on earlier planets: what is possible?

Wrecks often contain unrefined nuggets or refined resources, most commonly things like laterite or carbon, but sometimes a rarer material, even one that may not be normally obtained on the planet. What I want to know is: do we know the likelihood of finding rarer, non-native materials? And do we know whether there are any limits, such as some rarer resources being possible on some planets but not others?

For example, is it possible to find an iron bar on Sylva? If so, what's the likelihood? What about titanite/titanium? Same questions, but for Desolo, etc.? Which planets could have a wreck with lithium? And so on.

Partially asking this to try to determine whether getting 2 iron bars & 2 titanium bars from the EXO Request platform, when in this save I have only been to Sylva and Desolo so far and thus should not have either of those metals yet, will kind of ruin the progression, or whether it would still have been possible if I got lucky.