Perhaps the most important lesson for Putin is the realization that he cannot rely on the military’s assessments of its readiness, forcing him to take a more direct role in overseeing strategic operations. Ultimately, the combination of strategic lessons and a battle-hardened force will make Russia more lethal on future battlefields.
Russia is facing a US-NATO backed army while Murica has been doing nothing but picking on nations with much weaker armies for decades. Lots of US-miliary mercs fighting in Ukraine were shocked that it wasn't another cake walk.