MPG steadily decreasing. (Slowly)

A little background:

I bought an 06' gen 2 prius about 6 months ago. Had 109k miles. No engine lights and original hybrid battery. Only thing was cat got stolen and replaced, and cat shield installed which I assume affects mpg with SOME drag and weight.

When I first got it, I was getting about 47/48 mpg on display and 44mpg actual. (Average gallons I filled to miles. I took photos each time and recorded.)

I've put nearly 10k more miles on it.
I got maintenance don't regularly, 0w20 oil, did a transmission fluid change. Not much else.

In the past month or so, the P1121 light came on (coolant control valve) and I haven't had time to fix it yet but will soon at a mechanic. I topped off the transmission fluid with oem toyota pink for the time being.

Since then The DISPLAY mpg has been going down to 45.1 very slowly but steadily by 0.1 and never goes back up. It will always hover +0.1 / -0.1 then ultimately stay at -0.1 for a few weeks. Then drop again. My actual mpg also went down to about 42 +/-1

I was driving in the winter before but in california I can't imagine mpg to be affected by temp.. if anything I should be getting slightly more mpg now that it's warmer.


Question:
Is the p1121 causing the slow mpg loss? Or is it the hybrid battery? Maybe something else? I know 42mpg is still great but with only 118k miles at this point, if I have to replace the battery I'd rather do it now since I'll get the most use out of it. I'd rather not wait until the car has another 50k miles and getting 30mpg to replace the battery. (Lifetime warranty greenbean right?)

I did get like 30lbs fatter since then... could that also cause it? LOL

Thanks for any tips and advice. Car drives absolutely amazing. And willing to spend money to fix it until otherwise.