What you can get away with after you hit an opponent's shield really depends on who you're facing and what their habits are. If you're facing an opponent that only shied grabs, then you can get away with any option that will come out faster than or exactly on 10 frames (after you factor in your "on shield" disadvantage). But if you're facing Lucina, who has a 5 frame Uspecial, then you have to be MUCH more careful. A lot of top tiers have quick aerials OOS so you should be wary of those as well. Also, if you notice that your opponent likes to drop shield and attack with an A button move then you can pretty much do whatever you want.
So Nair's sweetspot is -2 on shield and its sourspot is -3. If an opponent tries to shield grab you, they will lose to spotdodge (frame 3 plus your 2 or 3 frame disadvantage so it's frame 5 or 6) or jab (frame 8 or 9). If you land the sweetspot AND buffer Dtilt (assuming perfect execution which people don't always do) then you will be even with shield grab (again, if your opponent doesn't waste any frames) and you'll still win since Dtilt will interrupt the grab. Most of the time Ylinks get away with Dtilt though since it's really hard to react to, especially online. SH Nair and Bair are also good options. Nair will come out on frame 9 or 10 (3 frame jumpsquat, 4 frame Nair, and 2 or 3 frame disadvantage) and Bair will come out on frame 11 or 12.
Fair1 is -3, Fair2 is -2 and Bair1 and2 are -3 on shield so all of the other I mentioned apply to them as well.
Also, you mentioned trying to grab after landing Nair. I would only recommend that if you think your opponent will stay in shield after your Nair. For instance, let's say you've been punishing him for dropping shield too early after you Nair his shield and he decides to wait out your options. Instead of doing Dtilt you could grab to mix him up. This is really the only safe scenario for Grab after Nairing someone's shield since Ylinks grab is so slow (frame 12 and after shield disadvantage frame 14 or 15).
Here's a google doc that has both frame data and shield advantage info. It's really helpful in figuring out what your character and what your opponent can get away with on shield.
https://docs.google.com/spreadsheets/d/16fmsoqDoQaR1eteVk2uuzIH2DB4iQHVrqiG8VRbRA7Q/edit?usp=sharing
This google doc lists every character's fastest OOS options. It's also pretty neat.
https://docs.google.com/spreadsheets/d/1wohzlhHm6bHbzl9bNBHiIQhjwzp9ZgoeTEostOMd3G0/edit?usp=sharing