Credit card churning is a bad idea
Credit card churning is the act of signing up for a credit card for its rewards, then cancelling the card once the reward is claimed, ideally before the card's charges fees. Common in the USA, and mostly absent in Europe and Australia where credit card fees are lower.
I believe that most people with ADHD should not do this. Credit cards typically require a phone call to be cancelled. Those with ADHD have trouble making phone calls, and are highly stressed by upsells and social savvy. What will typically happen is the credit card goes un-cancelled, incurring yearly fees that outweigh the benefits.
What's more, credit card companies will often make cancelling difficult. For example, the American Express website does not list a phone number to call, instead telling you to call the number on your card. If you have lost the card, you are therefore unable to make the call!

What to do instead
I feel it is better to sign up for ONE excellent long term credit card with an excellent points program and simply collect miles over time, so that your normal behaviour collects rewards. For that you may want to look at The Points Guy.