I can only imagine all these language had a word for 'uncountable number' which was then repurposed as mathematics developed (you need to invent column placeholders and powers of 10 before you have a specific notion of '10,000')

億 is just 10,000 * 10,000, perhaps you could say "a myriad of myriads" if one myriad wasn't enough.

Wikipedia article on 10,000 has this to say:

Many languages have a specific word for this number: in Ancient Greek it is μύριοι (the etymological root of the word myriad in English), in Aramaic ܪܒܘܬܐ, in Hebrew רבבה [revava], in Chinese 萬/万 (Mandarin wàn, Cantonese maan6, Hokkien bān), in Japanese 万/萬 [man], in Khmer ម៉ឺន [meun], in Korean 만/萬 [man], in Russian тьма [t'ma], in Vietnamese vạn, in Thai หมื่น [meun], in Malayalam പതിനായിരം [patinayiram], and in Malagasy alina.[1] In many of these languages, it often denotes a very large but indefinite number.

https://en.wikipedia.org/wiki/10,000

TBH, when I saw the term "myriad" in the title, I was expecting the term "八百万" (yaoyorozu, 8 million) [0].

[0]: https://en.wiktionary.org/wiki/%E5%85%AB%E7%99%BE%E4%B8%87