Apparently, i was trying to use the wrong command, to do something the game doesn't allow. For what it's worth, i ended up using the /execute command to run data on dropped items. My thanks to Mr. Röling for his help.

Ex: execute as @e[type=item,scores={tool=3}] run data merge entity @s {Item:{tag:{CanDestroy:["coarse_dirt","gravel","sand","snow","snow_block"]}}}

The result will be 0 on any normal calculator. You'll need something like Java's "BigDecimal" to even have a hope of calculating it. And even then it might just be 0.

This question goes way deeper than you probably expect:

• There are many different banner patterns. There are limits on everything, but you still have (in theory) 3 150 000 000 000 000 different patterns to choose from.
• You can name items. With the full Unicode range this gives you 1 114 112³⁵~4*10²¹¹ different names.
• This is all nothing compared to written books. The number of possible written books is more difficult to determine, because you have a line length limit, but it's probably somewhat around 10^70 000. But you can't actually use most of those, because most will be so dense with Chinese characters that they actually cause an error in the world saving process, which can make worlds unjoinable or reset chunks, among other things.
• Maps can encode big areas of a world, each with a lot of different blocks in them. The more I progress in this answer, the less sure I am of my numbers, but a wild guess puts the number of possible maps on the rough order of 10⁴⁰ ⁰⁰⁰, potentially a lot more. It depends on what of the world is actually stored in them.
• You can fill shulker boxes, each slot can contain everything except a shulker box, giving you a number of possible items that is probably somewhere in the area of 10² ⁰⁰⁰ ⁰⁰⁰.
• And who knows, maybe there are some bugs that allow you to put shulker boxes into shulker boxes or to get chests with content or to use structure blocks or whatever, or maybe your question already includes those Creative-only items. If that's the case, then you could get a theoretically infinite amount of different items. Then it becomes a matter of implementation how much NBT in one item still works without crashing the game.

So my final answer… doesn't fit into the character limit of a StackExchange answer. And I can't even post it into Pastebin, it's too long for that as well. So I'll round a tiny bit and say that your final value for p is:

0