Frig & freezer no longer cool very well

I have an older Maytag frig that suddenly doesn't cool very well - frig section & freezer. I first noticed what I thought was a noisy defrost timer. I removed it and the leads tested OK (continuity) as I manually advanced the timer. I reinstalled it and it does turn (rotate) as the motor runs. Still, it may mot be cycling properly - I dunno. When the compressor is on, the copper outlet line feels cold.

I manually turned the timer to the "defrost" mode (comp. & fan shut off) and it remain in that mode for ~30 minutes. Then it cycled back to cooling mode (comp. & fan kicked on). I now think the defrost timer is working OK.

The bottom shelf of the freezer has some frost buildup - never did that years ago. The "ceiling" of the refrigerator compartment has had water droplets hanging on it for days. There is also a bit of unmelted ice in places.

I put a cup of water in the freezer yesterday and it has a thin ice layer on top; the water's mostly unfrozen.
My own gut suspicion is that the defrost mechanism is not working right. If this is so, this might explain why the freezer isn't freezing. But would this also explain why the frig section is not as cold as it should be?

I wish I had a diagram that would show how to get to the defroster (heating coil) so I might check it.

In thinking more about this, a better way to describe the problem is that - ever so gradually over the past several years - this frig has seemed to be running a tad warmer. Ever so gradually ...

I wonder if that sheds any light.

I realize this is hard to diagnose from where you're sitting but wondered if anything came to mind. Any gut hunches?
I'd love to get her running again.

Any help is appreciated.