Renewables-hydrolysed hydrogen stored in the caverns where we currently store methane is quite likely a big part of the interseasonal energy solution.

I had thought that, until the technology in that link above came out. Ultra cheap thermal storage in dirt, if it works, looks much superior to hydrogen for seasonal grid storage, and has no geographical constraints. Hydrogen really wants salt formations to make cheap solution mined cavities; piles of dirt can be made just about anywhere.

Storing heat in dirt will only work if it's combined with low grade heart distribution networks.

In general though low grade heat is almost entirely worthless, all the Bitcoin miners and data centers that regularly just blow low grade heat into the air.

What? No. Here we're talking about storage of heat in dirt to be recovered as electrical energy using a steam cycle. This only requires moving the high grade (600 C) heat to the power plant itself. There is no need to move low grade heat, except the waste heat from this power plant.

Or indeed methane generated via the Sabatier reaction using hydrogen generated from renewables - higher energy density, easier to handle, store and distribute. And it can be fed to existing gas peaker plants, a known technology.

The problem is where do you get the CO2. This adds complexity and cost and looks to make the economics inferior to just storing hydrogen.

Indeed possible and both will probably happen.

If you put your H2 hydrolysers and turbines or fuel cells near the H2 store then handlng and distribution becomes easier. Move the electricity not the H2.