You also have the Transhab concept, which would also allow for the
volume contraints for the manned portion.

That would definitely be a good idea for the manned portion. If people
are supposed to live and work on the moon for several weeks to months,
they will need some room. But the manned portion would be a separate
module.

For that matter, I think you might also be able to manage a similar
form of inflatable tankage. There is no rule that the tankage has to
be metallic. There are several bladder systems that would work fairly
well.
Of course you could do this. But it is not necessary for the first
mission since 5m diameter is more than enough to store lots of
propellant.

Might be a good idea for an orbital propellant depot though: Two
bigelow 330 modules adapted to propellant storage could store more than
100 metric tons of liquid methane and more than 300 metric tons of
liquid oxygen. That should be enough for several very ambitious moon
missions.

And with such a large diameter, an almost spherical shape and many
debris protection layers, boiloff should be minimal.