What to do with all the riches PCs gather? Sacrifice it. Offerings of gold, gems, and other forms of wealth are traditional in many religions (these still exist today, albeit with a different veneer like tithing in various churches). A gold statue of the god, a gem that reflects their nature (such as a pearl for an ocean god), etc might be an acceptable sacrifice.
Please note that 25gp is the minimum the spell accepts. The DM might be amenable to a suggestion that more valuable offerings lead to better results. I would envisage a bolt of lightning descending from the sky to immolate the offering bowl on a success.
No, this is not necessarily an evil practice. Many ancient religions sacrificed one's own blood as an offering to the gods (good/lawful). The blood of a valued beast was also well regarded (neutral). Of course, sacrificing enemies or unwilling sapient beings is the realm of the vile (evil).
Historically, this can be seen in the religion of the Aztecs, with blood sacrifice of enemies being common. In ancient times, druids in Britain were known to burn victims in giant wicker birds, as well as ritually sacrificing beasts on altars and creating curse tablets to sentence people to death. Romans and Greeks often divined the future via the entrails of a sacrificed beast (with a diviner known as a haruspex).
This is distinct from treasure as it covers materials that should be wrested from enemies. Swords, armour, shields, etc taken in battle could be appropriate offerings to gods of wrath and war. On the evil end, the body of an enemy hero personally slain might do for Talos.
4. Food and Wine
Usually more appropriate for less bloodthirsty gods, food and wine are still common offerings in many religions today. Food would include seafood for an ocean god, rare spices, cooked meats, and so on. Anything uncommon and expensive is key, but also anything sacred to the religion (for example, wheat for an earth god). Wine would include other alcoholic beverages, or milk for teetotaller gods. Most gods in many religions tend to be heavy drinkers, though (I do wonder about this).
Jupiter/Zeus accepts cinnamon, wine, cakes/pastries (made with wheat and salt), cassia, meat, beans and greens, onions, leeks, scaled fish.
5. Cherished Personal Items/Loved Ones
This might include locks of hair, family heirlooms (not necessarily valuable), fingernail clippings (usually in religions such as voodoo), flowers/trees raised personally, a childhood treasure, etc. For a god of storms, it might be wood from a lightning-struck tree kept by the character in a locket since youth. The key is the emotional connection with object or person.
Historically, at the extreme end of the spectrum, this would include the wives, daughters, and sisters of kings. Many Greek myths describe such occurrences (naturally, I do not support or condone any of these ancient practices or beliefs), such as the legend of Andromeda being offered to Poseidon.