Question for woodworkers- repair before clear coat sealer help please

Submitted to Community Café


Hello to the woodworking experts here!  
Which filler product would you recommend to fill the split wood slice cavities seen here?  On this particular piece it’s about 3/8 inches thick and I was hoping to paint the filled repair to disguise.
Here I tried air dry clay and that seems to be a fail.

In the past-I feel like some of the proprietary filler products seem to dry chalky and crack during dry time and wondered if that continues long after the cure.
Would a caulk on top help with that? Or a wood glue coating?
Wood glue and Durhams?

Also what is the secret to store these commercial wood tub fillers ( or toothpaste type tubes) that tend to dry out in the sealed tub/tubes between uses? I’ve tried placing a moistened sponge under the cap. Maybe once they’re open up they just have a shelf life?  Would freezing -like when one does with used paint brush in a baggie -be advised?

Maybe I need to buy the smallest size available for my more infrequent- a handful-of-times-a-year uses.

Thanks for any tips you might have to offer. 👊🏻