Hello,
I hope everyone is doing well tonight. I just wanted to get some insight on how everyone with multiple cards manage them and keep payments in line, credit limits, due dates, open dates, and so on.
I am kinda young (24) and have several cards with a total limit of 105k (income of 39k). I owe nothing as I pay them off each month (I don't spend more than what I can afford, most of them I have open are from signup bonuses, go me!!!!!).
I keep an excel sheet with the cards name, last 4 numbers, open date, annual fee, signup bonus, bonus redeemed, payment date, credit limit, and a spend requirement for signup bonuses. This works out decant for me, but want to get ideas on how to make it better or an idea on what you all are doing!
Thanks for your time and I appericate everyone who helps 
I do a couple of things to assist me.
First, I have all my credit accounts linked to my online profile with USAA, which gives me one screen where I can see 'everything' at the same time. The second thing I've done is to have all my due dates changed to the middle of the month... that was the thing that made the biggest difference for me, honestly.
I like to tell folks there is a perfect system; it's called... the one that works for you! 
I'm the same age as you! Haha, you have way more credit than me though. But as for your question, I use Mint as well, it's been great, but I'm paying attention to Mint itself less, and moreso have been using Mint Bills (which is a separate site, separate app, same company) they have a section in he app to see just your credit card accounts, as well as all bills like insurance, cell phone, electricity, any recurring bills. If you can't link your account you can at least manually enter the bill, but I've been abe to connect most of my accounts, only manual bill is rent. It's really useful and helps me stay on top of where my money needs to go and never forget to make a payment. Highly recommended