Have you tried pine? How much do the gaps vary over a years period?

Can only fix that wide a gap with wood. The fix will never look good, but it will extend the life of the floor.

Just asked the floor refinisher about filler cut strips. Fastened is a great idea - The floors are easily accessible from the basement as they are the ceiling down there

Fastener*

Remove the two boards and replace with a wider piece of yellow pine. Yellow pine color changes pretty fast. It will blend in no time. You can always water pop it with a little baking soda mix to get the new boards darker.

Old houses have to be accepted as old houses. It’s not a modern ecosystem that you’re in the middle of inserting modern solutions into.

The traditional fill for the gaps in old floors was oakum or jute and shellac. Because wood moves in response to humidity, and keeps moving. To lessen that movement, control your humidity better. Big gaps in winter mean your house is too dry.

https://www.thisoldhouse.com/platform/amp/21016795/how-to-fill-gaps-in-a-wide-plank-wood-floor

We had pine floors in the kitchen in our old 1920 house. We were told that pine was used in kitchen only because they kept the fancier wood for the public rooms. We refinished the floors when we remodeled the kitchen but they quickly turned back orange and the filling was coming apart real quick too. We didn't mind because it was a kitchen, and old kitchens had lesser quality materials, so it was normal and part of an old house charm.

As others have posted, cracks between the floorboards are just another verse of the song when you live in a century house.

Mine has some. I live with them, and remind myself now and then how grateful I am to John Thurman for inventing the vacuum cleaner in 1899.

You may disagree, but I think my urepaired, well-aged floorboard cracks are less obtrusive than your filled one.

Pine is a closed grain wood. Filler doesn't stick readily to closed grain woods. Filler doesn't stick well to things that are moving. I don't know that I'd recommend replacing any boards since part of the t&g is lost doing so and unless you get old Pine to patch with, it's never going to match. If the current filler can be removed I would suggest you try to secure the boards that are moving and buy some Woodwise (or equal) prefinish filler. It comes in different colors and you'd probably need two or three to mix to get the proper color. It won't solve the problem but it's flexible and has a better chance at staying in the cracks. It's water based so it stays soft and can be resoftened with water as long as it doesn't dry out completely.

... but most of the time I tell people not to fill cracks in Pine unless the floor is completely sound.