I put my top tips here after about 15 years of collecting the best tips from hundreds of GMs:

https://slyflourish.com/top_advice.html

• Let the Story Unfold at the Table
• Set Up Situations and Let the Characters Navigate Them
• Be On the Characters Side
• Use Tools and Techniques that Help You Prepare to Improvise
• Focus on your Next Game
• Build Your World, Campaign, and Adventures from the Characters Outwards
• Pay Attention to Pacing
• Focus on the Fiction First and Mechanics Second

The OSR podcast Between Two Cairns does critiques of old school adventures.

https://open.spotify.com/show/6jxIIEDsjAfm5H6n7kcn8f

The Black Flag SRD also has some simpler spellcaster NPCs.

https://koboldpress.com/black-flag-reference-document/