Structurally sound(est) way to fill holes?
I got some new treadles for my loom and it turns out the holes on them are at different spacing compared to the old treadles. If I want to use the old and new treadles together I will need to fill most of the holes on the old treadles (I prefer the hole spacing on the new ones) and drill new holes that overlap the old holes by varying amounts.
The question is: how do I fill the holes in a way that is as structurally sound as possible and doesn't cause issues when drilling the new holes?
Looking nice would be a bonus, but not the first priority here.
The treadles are made of (Finnish) birch, probably Betula pendula, less likely Betula pubescens.
The second picture shows the forces acting on the treadle: at the end on the left is where my foot pushes down on the treadle, the tie up cords attach to the holes in the middle pulling up on the treadle, and at the end on the right is where the treadle attaches to the loom frame. The forces are about 10 kg for the ends (22 lbs) and twice that much for the holes in the middle (combined, so something like 1 to 2.5 kg (2.2 to 5.5 lbs) per hole, depending on number of shafts in use).
Would simply using wood filler work? I'm a little concerned about getting the filler into the hole wihout leaving any voids.
Or should I go with a dowel or a (long) plug? For a plug I would need to drill the old holes bigger first. I'm worried that would weaken the treadles too much.
A combination of filler and dowel/plug?