West Six10 cures to a light amber color, has a nice thickness to work into cracks, and can be mixed in small amounts directly from the tube. A transparent filler can be less noticeable under the gloss of the varnish.Another option is to fill it full of straight epoxy. Mix some up in a small cup. Let it gel then force it into your gaps.
This! You can get an excellent color match. West System has a good article about it (scroll down to "color-matched fillets"): https://www.epoxyworks.com/index.php/bonding-with-fillets/You can use white microballoons and sawdust, mix it with epoxy and try to find the color that you like to have. The color will not change while the epoxy is curing. Oh, and try to find a UV stabilized clear epoxy.
Your right sanding dust not sawdust.Fleetwood said:+1
Use sanding dust, not sawdust, add to the epoxy, then mix in some West 407 or 410 (both brown) and a white one to get the shade you need. It's a bit trial and error since as mentioned using sanding dust alone generally gives a too dark filler.