So... After some consideration, and your input, I think the solution for me is to have 3 "layers"
One is a simple layer. 1 gp = 10 sp = 320 cp. This is pretty easy to remember (I hope) and should be manageable for almost any player. Because there is a "daily expense" system, the players will rarely have to deal with petty change. Why 320? well that's the 3rd layer, hold your horses!
Second layer is the "unit of account". In the early middle age, you might buy a piece of land for 200 pounds... but you probably didn't pay with 200 pounds of silver! The pound is a way to count value/wealth, there aren't that many pounds of silver lying around. Later on you could get a pound in gold coin form (the guinea)... sort of. The guinea was worth 21 shillings (and sometimes more), not 20 (1 pound = 20 shillings).
In my system the "unit of account" will be the rupee, which is a large silver coin worth 2 sp... except said coin is quite rare. So when the party do a quest and collect their 1000 rupee reward, it will probably be paid in a mixture of gold and (ordinary) silver pieces.
I'm not sure how layer 2 will go. I'm hoping that after a while the players will "think in rupees" but we'll see. I may abandon it if it doesn't work out.
The "3rd layer" are the ugly/fun details, which will rarely come up. It includes the other coins and their weight/value. Historically the gold to silver ratio was 1 to 15, not to 10, so the standard gold coin is smaller that the silver coin. This isn't historical but it makes things easier on the player. The most used coins by merchants and the wealthy are the "standard" gp and the sp (the half ruppee). A lot of surrounding countries also use almost identical weights in coins so they are accepted. The rupee is a rarely produced, large silver coin worth 2 sp, with a lot of prestige attached to it. It also is symbolic as it is the traditional wage for a day's work - to earn your rupee so to speak. The Mohur is the gold equivalent of the rupee, and is worth 15 rupees, or 3 gp, and is also a rare and somewhat prestigious coin. The rupee is divided in 16 annae, a small tiny silver coin, often debased with tin or lead. 4 coppers = 1 anna (so 1 cp = 64 rupees or 32 sp). The copper piece, the paise, is more trusted by the commoners than the annae, but disliked by the rich because of its weight. A significant portion of copper pieces in circulation are cash coins from far away Xinjiang, these are considered to be the save value as the paise. I'll note that reducing the value of a copper coin is historically desirable because the sp to cp ratio was more in the 1:80 range. Finally there is the pie, a small copper coin - or maybe a bigger lead one, not sure yet -, which is worth 1/3 of a cp. These coins have a very small value even for the poor, but frequently given to kids. Also, a pie will by you a cup of tea, and that's always nice to have.
This third layer waaaay too complicated for usage. Occasionally however the coins themselves are part of the adventure. Maybe the players do something for a xinjiang trader and are rewarded 100 000 cash. Maybe the party raids a counter-fitting operation and now have 20 000 annae. Etc etc. But 9/10 adventures, it will be gold and sp found and rewards counted in rupees.
