Long straight dado?

I am finishing up rebuilding a barn/shed on family property to make a
small apartment.

The original first floor (about 25 years old now) was 2x6 tongue and
groove, laid over 4x6 on about 3 foot centers. I had planned to sand
the wood floor and put several coats of poly on it. The wood has some
character and will look OK.

This building is on the shore, and was knocked off its pilings a few
years ago. I jacked it back into place and bolted it more thoroughly down.

My problem is where the edges of the 2x6 come together on the top
(inside) there is a gap between most of the planks of between 1/16" and
up to 5/16". I had originally thought about filling the cracks with
epoxy and a filler, then decided it would look like shit. I've tried
cutting tapered splines to drive in the cracks, but some of them
(cracks) vary so much in width over a 14' run, that I am afraid of not
having enough embedment to be leaving wood in the crack after sanding
the floor.

I thought of routing the worst of them out to say 1/2" wide by 1/2" deep
and laying a spline of a different color wood (I have a lot of
Australian Hard Cypress left over from another job, and the upstairs of
this one). I thought that would give an effect sort of like a teak and
holly sole on a boat

I am not all that conversant with a router and the first test groove did
not come out well. I tried cutting the 1/2" x 1/2" groove in one pass,
with an Oldham Viper bit, the box mentioned starting from an exposed
edge. I have an old Craftsman 6 amp router that says it is 25,000. rpm

From reading other posts, I see it might have been better to set my
"fence" up on the left side of the groove, but what else do I need to
do? If I have to make three passes with a different depth every time it
obviously makes the job that much slower....

I had thought to run the bit down the center of the existing crack, thus
taking some meat out of each side, but might the differing
characteristic of the different planks affect the cut?

Are there Dado kits for skilsaws (7.25 circular handsaws)?

Any other ideas?
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