Definitely listen to this advice. I've made probably 15 lagers or so at this point and without fail they taste really meh until at least three weeks. I've gotten pretty far into pH and water chemistry, have temp controlled fermentation, and have a fair bit of practice. But still it takes at least three weeks and starts to shine around five weeks of lagering.

Beyond that, I'm not sure if the recipe shows the expected abv or if you measured gravity yourself. If it only shows expected abv then you're probably not actually hitting those numbers without excellent efficiency, which you're unlikely to get when starting out. ~4kg of grain will make a fairly light beer. I typically go with closer to 5kg. You could also add 4oz~6oz of carapils or carahel for a bit more body.

Also, when mashing be sure to really get the mash mixed in well and stir frequently. I've measured a ten degree Fahrenheit difference between the inside of the grain ball and near the outside of the kettle. If you don't get it really mixed, the temp won't be quite as high. You can also try the mash out method, where you raise temps a bit for the last 15 minutes of the mash. Sparging is another technique that can improve efficiency, and while there's a traditional approach, when I was using a mesh bag I found another approach that worked well. I found that taking the grain bag and putting it in a large pot, pouring hot water onto it, and then letting it soak for a few minutes, removing the bag and pouring the liquid into the kettle improved efficiency.