Hello thor

Using filler to make cosmetic fixes to gaps will only make the gaps look worse in my opinion. Dark wood is easy because the epoxy will fill the gap and will look dark as well. Lighter colors show gaps easily. Fillers will show up as different colored streaks and lines. Unless you match the colors perfectly and anticipate the color chang due to wetout closely as well. Also the fillers and wood will age differently and therefore change colors with age differently.
There are a lot of fillers and mixing options but i cannot make them work to my quality expectations. I no longer use anytype of filler but i do spend plenty of time stripping. Thats the only way i have found that resolves the gap problem.
Gaps the thickness of a business card are not a structural issue, they will fill with epoxy and be glassed over.
I can repeat some good advice, stand back 10 feet, take off your glasses and then see if the tiny gaps are still visible. With your face 6 inches from your work for so long you get over sensative to small, small, flaws. You are the only one that knows about those gaps.

Search this forum for "what is a gap really?" My thread with your question.

Can you see the gaps in this boat?

Robert, Visionary