Polyfilla vs gypsum, which is tougher?

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Is polyfilla just gypsum with another name? I want to fill the gaps between my floorboards (which will then be painted), I've already packed out the gaps with corrugated cardboard glued in position. I want to just 'wipe' a top layer of filler into the gaps to make them nice for painting. I know gypsum isn't really tough but if it's recessed slightly would it be ok, or would polyfilla be a better choice? I don't want to use that 2-part resin filler. I accept both will crack a bit here & there with movement.

Could you use some kind of paintable sealant, it will flex with the boards and not crack.

Not sure how well painr would adhere to sealant but it's also very messy to apply. I prefer filler for a neat finish.

they are not the same, but neither is suitable for floorboards as they will crack and crumble.

Caulk might do, and you can paint it.

I've never been able to paint caulk, I have dark areas all over my house where I have used caulk and not been able to paint over it, the paint splits. I then use a different paint which does adhere but leaves a darker finish than the rest.

Well, on piglets advice I'm trying caulk. I'll report back how it goes. I think/hope the key to painting will be to ensure the caulk is properly dried out first.

silicone sealant might do, if you choose a colour similar to the finished colour of the floor, or darker.