[photochemsyn]

The only mechanisms at the CDR Database that look like actual long-term offsets are in the mineralization category, meaning the production of carbonate rocks from atmospheric CO2. The problem is you need the counterion to make these rocks, i.e. calcium or magnesium, and these are often already locked up in rocks. Mine tailings have been proposed as a source, I don't know.

https://en.wikipedia.org/wiki/Carbonate_rock

The sci-fi approach that would be great is pulling atmospheric CO2 out of the air and making diamond from it. The technology is sort of there already but the costs are ludicrous. The pathway is atmospheric CO2 + water -> methane -> synthetic diamond production. That last is a slow energy-intensive process, but such diamonds would be easily distinguishable from the 'real' geologic diamonds (as they'd have the same amount of carbon-14 as the atmospheric CO2 does). Worth someone writing a business plan I think.