Do you guys have any advice for grabbing incoming tsuki fists?
You cannot grab an incoming tzuki fist. First you need to deflect the punch with an open hand. Keeping that hand close to Uke's fist, then only you can grab his wrist. My advice is to time your stepping back with Uke's punch/step. If you move before he does, you become a moving target and he may get you when you are not rooted, and If you move faster than the strike, you may create an opening for the Uke to re-direct the strike.
If you move directly backward in a straight line as a karate-ka would do, then you should do what a karate-ka would do after the block, i.e. an uraken (back fist to the face). But, of course, we do not actually contact in aikido; it is just a feint to distract him or cause him to raise his other arm (which by the way is not away on vacation as most people would assume
How you step backwards depend on your kamae. Nobody will tzuki/punch you for the sake of punching. You initiate that by offering an opening, sort of like inviting him to punch. If you in a hanmi kamae with your back foot behind your front, one hand in front at shoulder level and the other in front of your abdomen leaving little opening for a punch then, martially/logically, it is very unlikely that I will step forward to punch you. Every attack creates an opening that awaits a response and vice versa
The rest you can figure it out.
PS Do not move 45 degree backward. You are not exactly off the line of attack. In fact you are still in line of attack from the other arm. The direction should be between 5-6 O'clock. Don't take my words, work it out with an Uke.