Rusty wrote

I need to put rods through my drain about every year or so due
to dishwasher detergent congealing and clogging the drain.

It isn't the dishwasher detergent that congeals.

Using "Finish" (top brand ?) tablet detergent.

Same one I have used for decades now.

We rarely use the sink which drains to the same
path, so it doesn't get much "flush though".

I'm assuming when the dishwasher drains, it
re-congeals when the expelled "liquid" gets cold.

I don't believe that. I am currently running my
latest dishwasher on the 6' wide concrete slab
that runs the whole of the length of that part
of the back of the house, under the 6' eave
there just because I have been too lazy to install
it in the kitchen in place of the previous one.

So the drain is just lying on the ground,
so I can see that nothing congeals at all.

Any suggestions ?

Work out what is congealing.

At one time I used to tip the molten fat out of
the big stainless steel pan I do the legs of lamb
in and then just put the whole thing in the bottom
of the dishwasher. That approach does end up
with a lot of fat in the drain and that does
certainly congeal there.

I don't do that anymore, wipe the pan out with
kitchen paper and that has fixed the fat problem.

Maybe a liquid dishwasher detergent ?

I don't believe that is going to make any difference.
The solid pellets of detergent are completely dissolved
during the wash cycle and that dissolved detergent
doesn't ever congeal when its cold.