The Ryūjō din't count towards the tonnage-limit, she was below 10k tons which was a loophole in the treaty the Japanese used, that loophole was closed off in 1930 in London. But there is a very good reason the Japanese didn't build in sisters, it was a very expensive ship to build for very little fighting power.
Ryūjō was counted as part of Japan's 81,000 tons because as you wrote the 10,000 ton loophole was closed by the 1930 LNT. IIRC it was the Japanese who asked for it to be closed because Ryūjō demonstrated that an effective light fleet carrier could not be built on less than 10,000 tons.
And as for your last comment, no it doesn't require ASB.
My idea was that Japan would be in a stronger bargaining position at the Washington Conference because the country had a bigger Gross National Product. The Japanese Government would use some of its extra tax revenues to build up a bigger navy between 1895 and 1920-ish.
If the Japanese did get 5:5:5 that would allow them to maintain a navy that was 67% larger than what was allowed by the treaties IOTL. A bigger economy solves the problem of how the Japanese could pay for the increase.
It also allows Japan to maintain a larger Naval Air Force, Army Air Force, Army Ground Force (and/or the Japanese equivalent of the USMC or to created one if it didn't have an equivalent of the USMC IOTL), build up a larger merchant marine and build up bigger stockpiles of strategic raw materials and oil.
If the American Delegation walks away from the Washington Conference the Japanese would have had the money to complete the 8+8 Program. The American economy would still be several times larger than Japan's, but as the Japanese could build more the Americans would have to build even more back to win the arms race. Many American politicians and tax payers might think winning the arms race wasn't worth the financial cost. E.g. the opposition to Ronald Reagan in the 1980s.
Plus as there was no Washington Treaty the Anglo-Japanese Alliance did not have to be cancelled.
All the above requires a Japanese economy double the size of the real world between 1895 and 1922. Which is probably ASB and if it isn't it requires a POD before 1900.