Am I able to remap action groups after I've launched?

Unfortunately, right now you can't. It has happened a lot to me, launching and then realizing I didn't remember to set up the keys to something. If you have to do something synchronized, you can try to pause/unpause the game while mousing from one component to the other to minimize the desynchronization.

The best solution to this problem that I have been able to find was the tiny mod Actions on the Fly which allows you to edit the action groups in flight mode.

It's tricky and difficult, but doable. You'll need a good text editor (e.g. Notepad++) and to know your way around the game files, in particular saves and .craft files

Keep backups of everything at every step, because it's very easy to corrupt the save, or replace something else than you should have.

If you have the craft's template saved (as .craft file) half the work is done, skip the next step. If not, you'll need to create it.

If you don't have it saved, first, dig out the craft from your save file. src Create a new (dummy) craft and save it, then open your save file in a text editor, find your craft in flight (be it by craft name if unique, crew members names, or a unique part name, or orbital parameters), then copy it - the part after its ORBIT { } block - into the .craft file for the 'dummy', retaining the headers of the .craft file but replacing the craft structure.

Next (or first, if you had the .craft saved before launching it), load the .craft file into VAB, fix the action groups making them as you want, test if everything is alright, save it. Open the .craft file, then find the ACTIONS { } block. Open the save file, find your craft in flight again, and replace its ACTIONS { } with the fixed ones.