to maintain you NASM certification, you only need CPR/AED but I would strongly suggest that you also get certified in First Aid, as you already did. The Red Cross is an excellent choice for this training.
You can also look into American Heart Association. Red Cross and AHA are both very reputable and good organizations. If you like Red Cross then just renew with them. CPR/AED are the ones you should definitely keep, but if you are willing to put some extra time and money in keeping your First Aid as well then you should do so.
I went with American Heart Association, which I really enjoyed. The particular one I chose was the Heartsaver First Aid and CPR AED course. It was bad enough you have to sit through a mandatory amount of hours, but the instructors made the classes really enjoyable, fun and informative. The also bundled the AED, First Aid and CPR classes into one which was really helpful, so I did not have to make multiple trips.
While we may only need the Adult CPR/AED training to maintain our certs, I recommend everyone take the full CPR/AED/First Aid course for Adult/Child/Infant. These are skills that could save a life anywhere anytime. I am a Red Cross instructor and have taken the American Heart classes as well. Either is fine and both will prepare you for almost any emergency.
But we should all at least mentally go through the steps for performing CPR/AED/First Aid from time to time. I review my manuals every two months unless I teach a CPR class within that time. (I review and prepare for each class unless I just taught a class no more than two weeks prior to my next class.) You don’t need a manikin to practice. A firm bosu works great for compression and you can simulate breaths without the manikin. Just rereading the manual is better than nothing.