Floor board painting advice

So we decided to have our floorboards painted in kitchen and bathroom (they aren't great looking boards). We got F&B floor paint of which the decorator put down two coats. He then advised varnishing, and I said yeah fine.

This was a mistake - it looked much better before and it now looks dirty even when it's clean. And realising in any case that a white kitchen floor is totally impractical, we are going to repaint them a dark colour, either dark blue or dark grey.

But also cracks quite swiftly appeared in the paint between the boards, which don't look great. We asked the decorator to fill the larger gaps which used to collect crumbs, so maybe he has used hard filler instead of flexible caulk? Although it looks like it's happening everywhere.

In any case, is there any way of preventing that happening again when we repaint? Cracks will presumably be much less noticeable in dark paint anyway.

In terms of what paint to use, what are people's experience of F&B floor paint? Is it tough and durable? Am considering using something like Dulux Weathershield, either satin or high gloss. Any comments?