NATO learned many valuable lessons from supporting Ukrainian forces fighting the Russian invaders. The most important lesson was that the new NATO members in eastern Europe were correct about Russian aggression and efforts to rebuild the Russian empire that dissolved in 1991. Ukraine was not a NATO member but wanted to be one and that was the main reason Russia invaded Ukraine, to keep it out of NATO as well as the first former components of the Soviet Union to be reabsorbed into Russia.
Other lessons were often simply repeats of past wisdom that had been ignored. This was mainly about underestimating the speed with which more key weapons and munitions would have to be produced. This was demonstrated during the 1973 Arab-Israeli war when everyone underestimated the speed at which modern weapons and munitions would be used or lost in a modern war. Israel pointed out that this problem was often ignored or played down because a solution was politically difficult and expensive. This problem is now harder to ignore.
The Ukraine War is also different because NATO cannot put troops into Ukraine as that would have given Russia an excuse to mobilize and declare NATO the invader. This meant that the NATO personnel most directly involved were intelligence and logistics specialists. This NATO was able to do, and learned some valuable lessons about the need for these specialists in the early stages of a war.
Another lesson relearned was that despite peacetime propaganda, if an actual war breaks out it will be revealed which country has the most useful weapons or munitions. This was certainly the case in Ukraine where many little-publicized non-American weapons were more in demand on the front lines. The U.S. has been reluctant to accept this but gradually has been more accepting and adopted foreign weapons systems.
Finally, the need for a larger “rapid-reaction” force of combat ready troops is definitely a priority. Currently NATO only has 40,000 troops available for this. NATO should have five to ten times more of these troops. New NATO membership candidates, Sweden and Finland, realized this before the Ukraine War began and were seeking to form their own rapid reaction forces, realized after Russia attacked Ukraine that there is safety in numbers, and that the new Russia made it very difficult to be neutral.
These have been valuable lessons for all NATO members but it will still require some effort to implement solutions after the Ukraine War is over. After all, there is still China. While not bordering NATO countries, Chinese aggression in the Pacific is a threat to all NATO nations.