I maxed out Lahni a couple of months ago purely through playing - I didn’t craft any cards using Scrap.
Shockchain was the very last one I needed - a rare / blue card.
By the time I got it, I had something like 33 or 34 extra gold / lengedary cards.
For Keegan, I think it was Venom Boost I needed - epic / purple. I had 31 extra gold / legendary cards by the time I got it.
About 2-3 weeks ago, I still had 1 or 2 cards left to max out for some of the non-GOW characters - Kat, Grace and Emile (Sarah Connor was maxed by this point) and the Horde-only characters (Kait, Fahz etc) and in the end I decided I couldn’t be bothered to grind anymore. I had around 38,000 Scrap and just used that to craft the remaining cards I need. I still had about 6,000 left. I mean, it’s not like there are any character skins to craft in Supply…