I think the mass ratio they mentioned is an extremely conservative
estimate. They have probably a 500kg allowance in their calculations to
allow for weight gain. So a payload of up to 500kg is probably
possible.

Another trick that seems possible using said inflatable materials is the
construction of a very light weight positive displacement turbo pump
equivalent. Kind of like a cross between a Flowmetrics pressure pump and
a turbopump, it should have most of the advantages of both. This might
be ideally suited to such small low cost launch vehicles.

Could you elaborate on that? Or is that idea confidential?

I don't really see what is wrong with turbopumps. If you have decent
margins, turbopumps can be very reliable and also easy to manufacture.

The V2 turbopumps were assembled under very bad conditions in forced
labor camps, and yet they managed a decent reliability.

And modern airplane turbine blades are operating in an extremely
hostile environment, yet they manage to work for many thousand hours
without major maintenance.