Terraforming Venus

[Rynn]

Trouble is, hydrogen is volatile and low density, so it is expensive to import hydrogen

At the risk of becoming a parrot (given how often I squawk this) Expense is a strange term in OA. It might be quicker to let your assemblers spawn a comet-capture fleet and crash water ice over a hydrogen mining outpost along with transports. But either case requires little input (at least in the mid setting, in the early setting there's still a lot of work to be done designing the protocols and equipment) and is really a question of how long you want to wait.

To add something beyond this hydrogen as a gas and liquid are very low density but metallic hydrogen has been reported as possibly having a density of 0.6g/ml IIRC. Given OA tech it wouldn't seem that far fetched to believe that mt-hydrogen could be mass produced, stored and transported.