Pikachu's chaingrab is one of the easier ones, but you can improve the percent window with tighter timing. It works on fox, falco, and falcon, and it's easiest on falcon and about the same on fox and falco, though a little harder imo on falco.
On falcon, the chaingrab starts at 0%, and that's a good goal to aim for if you want to say you have a decently consistent chaingrab. On fox and falco, the chaingrab realistically starts at around 30%, though with TAS precision and certain other parameters (that I won't get into) it can start at 0 too. Generally, though, at 0% against spacies I try to go for more consistent punishes and then go for a regrab into chaingrab if possible.
You never have to pivot grab to do pikachu's chaingrab, so all you need to do is practice dashing and jump-cancel grabbing as soon as you are able to move after the upthrow, and react accordingly to DI. You do have to jump-cancel, because otherwise you will be slower and also not keep your dash momentum, so you won't go as far. If you have 20XX it's really easy to practice, otherwise get a friend to DI back and forth on FD for a while with you. Have them spam shine or jump, and practice regrabbing slight behind DI as well as max distance DI. Slight DI is generally the hardest to get because you have fewer cues to react to and have to make more decisions (ie should I turn around, should I dash), so that's what better opponents will do.