You're right, there seems to be iron rich deep sea currents in some places that could be good opportunities for upwelling, especially if you had a carbon neutral way to do it.
There is also emerging technology for nitrogen fixing that could work - Electrocatalytic Nitrogen Reduction.
All this being said, if the goal is carbon sequestration building a viable business is going to be next to impossible.
The easiest way would be to use your renewable energy to pull bicarbonate out of the ocean directly with electrolysis, reverse-osmosis, whatever magic tech the wizards in MIT are cooking up.
Only then you've got the problem of what to do with it all once you've got it on hand.
[1] https://www.nature.com/articles/s41467-023-36992-1