From what little I've researched about privately producing back to the grid here in Finland, it really didn't make much sense. You get terrible rates and as you said have to pay the transfer fees too. It's priced in a way that they clearly would rather you didn't do it at all.

But the NordPool isn't really a system designed with tiny private producers in mind. Price goes to zero, or sometimes even negative, exactly to try to prevent having to pull electricity production down as that's expensive and complicated. It's clear to see that it isn't a sustainable model in the long run, but hopefully it incentivises companies to build the solution - storage - to make use of all that "wasted" energy and stabilize the price and market.

I'm not familiar with NordPool specifics but this is exactly right and is playing out in California and elsewhere too. Basically just the duck curve. Storage is all but required as solar covers 100% of midday load.