The 'pride' of Japan's space booster family today
is it's H2A...It can launch upto 10,800 kilos into LEO,
5,000 kilos into GTO [or 4,000 kilos to the moon?].

But...The H2A has only TWO strap-on solid-fueled boosters Castor engines].
Why not an H2B with FOUR strap-on solids?...and a core stage with stretched propellant tanks?

I can easily envisage such an H2B evolving from an H2A,
and it would be able to send upto 15,000 - 17,000 kilos
into LEO [about the mass of a Russian Salyut spacestation]
or 6,000 - 7,000 kilos to the moon [a two-seater circumlunar spacecraft].

And such an H2B booster could be made available in less than 5 years. If it had the will, Japan's government could fund a manned circumlunar mission that could take place within 10 years.

I wish somebody from Japan were reading this right now.
:-(