6 puck lights on their own dimmer not getting as bright, replaced dimmer but that didn't work waaat?!

Do you accidentally have them wired in series instead of parallel?

Also, if you just copy the wire placement from the old dimmer to the new dimmer, it is possible the connections are different. Notice on the controller picture negatives are on both outside. Positives are on the centers.

Can you bypass the dimmer and directly connect the input positive to the positive going to the puck lights and the input negative directly to the negative going to the puck lights and see what happens?

Do you have a spare puck light you could just connect directly to the dimmer? But after connecting the input directly to the wires, running to your puck lights that pretty much isolates it you could further isolate by going in the middle of your run of puck lights and disconnecting the first two or three and see if you can cut your problem area in half that way. For example, disconnect the last three lights from the chain and then try to power that section directly with 12 V.

Do you have a voltmeter. If not, you can get one at Harbor freight for a few dollars. It might be worth it to make sure you were actually getting 12 V on the input supply to your dimmer.
If you don’t have a voltmeter, a 9 V battery is a pretty good approximation to 12 V as far as LED lights are concerned. You could just touch the positive and negative leads of a puck light or puck light wire run to the positive and negative on a 9 V battery as a way to quickly troubleshoot.
I think you need to make sure you were actually getting 12 V at the input of your dimmer.

Maybe it was something with the connections. Sometimes they oxide over time and the resistance goes up.