I Joined a D&D Group and Took Over as DM After Two Sessions, Is That Too Much?

So here's my story. I've always been a huge fan of fantasy novels, so when my friend Mark invited me to his Dungeons & Dragons group last month, I was thrilled. It had only been two months since the first campaign ended with a climactic battle, and everyone seemed excited for more adventures. I'd never played D&D before, but I've read countless books on fantasy worlds, mythology, and RPGs. I thought, 'Hey, I might be able to contribute something here.' So, I joined in as an NPC character who was a librarian with extensive knowledge of ancient lore, hoping it would give me an excuse to drop some fun trivia into the game. After our first session, Mark (the previous DM) mentioned that he wasn't sure if he wanted to continue being the DM for the next campaign. I couldn't believe my luck! It seemed like fate was telling me to step up and take charge. Plus, there's something incredibly satisfying about crafting a narrative and watching others react to it. So, during the second session, when Mark expressed his doubts again, I surprised everyone by volunteering myself as the next DM for the upcoming campaign. Some of my fellow players were shocked but said they'd give me a chance if we could agree on some ground rules. We set up a Google Doc to work out ideas and logistics together. Now here's where things got tricky: after taking over, I started adding all sorts of intricate plot twists and challenges that required tons of preparation. My players seemed to like the complexity, but it also meant lots of prep time for me. Some sessions took up entire weekends! I've been running this campaign for a month now, and while most people seem engaged and excited by the storyline I'm weaving together, Mark has started sending me messages asking if he could have his old job back or take over again in a future installment. He feels it's too stressful trying to keep up with my ambitious storytelling. Am I being an asshole for taking on this role? Should I step down and let him resume DMing? Or is there a way we can work together to split the responsibilities? Your thoughts would be appreciated!