Here's another point of view, which I have explained to my players.
All prices are expressed in the unit of account; they are not literal coin counts. So when I say that you find gold coins worth 1,000 gp, that doesn't mean there are exactly 1,000 coins in the pile. It means there are some number of gold coins that are worth 1,000 "standard" gold coins at a money changer's shop using standard weights and measures.
The actual coins vary a bit by weight, alloy, and purity from kingdom to kingdom, depending on the mint that made them and whether anyone has shaved off some edges. But professional money changers won't be fooled, so no matter how thin or thick a coin is, it can be converted to "standard" weights.
But we hand waive that away mostly. It's an abstraction. No one is playing D&D to play weights and measures. Sometimes I throw out that you find "400 Golden Thanes", which are larger gold coins bearing the likeness some dwarf king; and they're worth 3 GP each. But I only do this when it's relevant to the adventure for some reasons, such as "Oh, he was paid in coins from the Dwarf King, maybe he's an agent of theirs?" or something like that. Or sometimes I do it for flavor.
But then my players ask "And how much are they worth?" and I say "1,250 gp". I'm not insulted. They're just trying to keep their loot record manageable and I'm not trying to make their lives harder than it has to be.
A couple other things I do...
One of my tropes is that dwarves and gnomes can smell metals, so PCs of these races can identify metals and alloys (assuming there is no illusion magic upon them capable defeating all five senses). And while PCs can only detect the ingredients ("These coins are an alloy of silver and gold", for example), the professional money changers in town have developed their sense of smell to the level where they can determine the exact percentages. And alchemists have the ability to quickly separate out impurities and produce "pure" samples. So that's why you can always take your trade bars and non-standard coins to them and get them changed out for "standard weight, 100% pure" coins.
I've established that merchants have "standard trade gems" of uniform size and quality. "Trader's Pearls" are worth 100 gp, and "Trader's Diamonds" are worth 500 gp. When you have races like Tritons and gnomes about it's easy to produce a steady supply of these, and they're used by merchants who travel long distance, so as to avoid having to lug about heavy chests of coins.