This has been mentioned before But we havnt heard back on the results.
I know that most of the post war housing commission houses where I grew up had plane square edge boards ( no t&g) & the gaps went all the way thru.
Yo gota fill the gaps unless you like distributed floor ventilation.
I have seen a floor that was done, pine boards & black filler with cleer estipol.
Looked great.
This was done many years ago pre no more gaps so I'm not sure what they would have used.
My money would go to one of the high end sealers.
Flexibility would be imperitave or the floor would just spit it out.
A bit of dip & bulge would be a minor detail in the greater scheme of things, I don't recon the vertical profile of the sealer would change much.
cheers