How to get puppy to be independent

I have a golden retriever that's 15-weeks old now, I got her about 4-ish weeks ago? My issue boils down to this: I cannot surveil her 24/7, I need her to entertain herself. That is to say, I can hang around the living room with her (or I can stay in my office quietly and leave her to her own devices) but she barks at me for attention.

Now, I've been told to exhaust her mentally, so:

  • She eats all 3 daily meals out of different foraging mats, and is given filled Kongs daily
  • I have a snuffle mat for her to do some nose play with, and a snuffle ball even to wrangle with for a treat
  • We do lots of training--probably more than I should be, honestly (maybe like a total of 40 minutes/day of training? Sit, settle, attention/recall)

And, she needs her physical exercise, so:

  • If weather permits, I allow her 10 minute sessions to just enjoy the outdoors in my enclosed backyard
  • I play fetch and tug with her for maybe like, 10 minutes

After all this is done, she goes into her ex-pen with access to water, some chews (Nylabones/Benebones), and a plush toy--we rotate out toys to keep it novel.

  • She has a love-hate relationship with Nylabones/Benebones as she cannot seem to keep them still so she'll growl and bark at them
  • She will ignore her toys to dig into her crate mat or wrestle her bed to bloody death

There's so much conflicting information out on the internet. I've seen everywhere from "play with your dog at least 2 hours/day" and "they only need 5 minutes of exercise per month of age" and "only train your dog for 15 minutes total/day" which begs the question; if I'm giving her 30 minutes of physical exercise (i.e: letting her loaf about in my yard or playing fetch with her) and 15 minutes of training, what is she doing for the remaining 23 hours in the day? Even if she's sleeping for 18 hours that still leaves 5 hours of... What, exactly?

That's not to say I don't spend time with her, I just need to know she will be happy if I leave to say, cook, clean, or go to school.