What app/tool should I use for simple design?

You can try out www.aiflyer.ai. You just have to describe what you want to the AI assistant, and it'll come up with the flyer. Pretty fast and easy, and you can pay for just one design if you need something for one-time use. This was what it came up with for me some weeks ago.

Everyone here hates Canva, but this is what Canva is for. Use Canva.

I hate Canva lol... I'm not a moron, and I can tell it's built for maybe... a moron. But if expediency is what I want, I'll use it. I want to start learning, and Canva seems like it'll keep me just dumb enough to never really know what I am doing and still using Canva. Should I just go AI? Been geeking and learning about models for a while. I'm getting better at prompts with GPT for copy anyway. Just want to place my learning time into the right tool.

OP wants to design 'all over shirts,' which is going to be very difficult in Canva due to no pattern tools and limited canvas size. OP, use a third-party vendor that will do the work for you, like a DTG shop.