I think there's a lot of question marks hanging over a lot of these things. I don't want to sound more confident in hydrogen than I actually am, and I don't want to sound more dismissive of syngas and other long term storage options than I really am. I'm really just not sure at the end of the day.
Another option that I think is under-discussed is just seasonal storage of biogas. Germany currently derives around 10% of total electricity production on any given day from the burning of biogas. If we could replace all of that daily usage with renewables + batteries, then we could potentially just save up all that biogas in reservoirs and burn it during periods of low renewable output. No new technology, and very minor new infrastructure needed.
It's hopeless to scale biogas production up to a point where it provides for the whole grid, but it might just be enough to deal with week-to-month scale renewable shortfalls.