As a long time DM (30+ years) I feel like a lot of experienced DMs are doing themselves and their players a big disservice by not reading the 5e DMG because "they have read it all before."
First, 5e is not the same game as any previous edition. You are making mistakes and applying rules incorrectly or incorrectly applying rules when you rely on your previous edition knowledge and experience to know how a wand works or whatever. Being the final arbiter of the rules is your primary function. You should probably at least make an effort to do it correctly.
Second, there are a lot of good bits in there that aren't obvious even based on experience. If you have been running the game the same way for 30 years, you could probably use a freshening up of your technique. Whether it is thinking outside the box in regards to optional rules, or digging into the designer's motivations behind the game, that information can only enhance your ability to run an engaging game.
Finally, you aren't the perfect DM. No one is. There are lots of sources online for helping you become an even better DM (even after 30+ years) but one of the best sources is right there on your shelf, and you haven't cracked it because the "What's a roleplaying game?" section bored you. Ok, boomer.