I would use something like SikaFlex self-leveling sealant. They should sell a textured product (contains sand) that will look more natural than the flat rubbery material.
I would also suggest removing all the old material, cleaning out the gap, and using foam backer rod to fill up the depth. The tech sheet on the SikaFlex should give a maximum fill depth. The lines should have no slope to keep the material from pooling. If there is a slope, then you’ll need to use a faster setting compound.
Thanks. I do have a lot to do.Sika makes all around great products.
If the OP has a lot to do, getting it in sausages is a lot more efficient.
No need to sell it … you can use it to decorate Christmas cookies and cakes. You just need the different flower tips to make the fancy designs.
Seriously, an acquaintance of mine does side work as a baker and she does cookies, cupcakes and wedding cakes off the books for friends and family. She uses her husbands electric caulking gun to do the detail work. She laughed when I asked if squeezing a pastry bag all the time made her hands hurt. She laughed and told me only amateurs use pastry bags. If she has to dress up a hundred cupcakes, it’s the Dewalt caulking gun all the way!!