I've used physical coins for players, too. Even if the treasure is not easily converted in game, you can have it be so in the physical currency. I did this in 4E, nonetheless, where high level PCs had treasure piles in the millions of gp.
All you need to do is give them additional currencies with higher values - and for 5E that is barely necessary. After all, treasure in 5E does not grow geometrically.
For a party of 5 PCs, I could get by comfortably with 50 Copper, 50 Silver, 50 Gold, 50 Platinum, 50 Mithril (10 Platinum to 1 Mithril - Shiny Silver Coin (each worth 100 GP), 50 Adamantine (10 Mithril to 1 Adamantine - Shiny Black Coin - each worth 1000 GP), 50 Crysteel (10 Crysteel to 1 Adamantine - Clear Disk - each worth 10,000 GP), and 50 Astral Diamonds (10 Crysteel to an Astral Diamond - Glass Gemstones - each worth 100,000 GP). That is like 5,555,555.5 GP - more than the party will ever have. 400 'coins'. Total - an easy amount to manage. You can buy some packs of plastic coins, a set of varied color plastic chips, or go get some cheap foreign currency.