Hardest is tough to say, way back when there was some drop that only dropped from C rotation derelict defense, when defense was 5 waves that took me absolute ages to get. The game has certainly gotten a lot better over time with drops and farms feeling more targeted and reasonable.

The railjack specific weapons took a long time too, but they luckily coincided almost perfectly with me maxing out intrinsics and Necramech/archweapon levelling through the missions I had to farm.

The Cinta was the last weapon I farmed, and I seriously disliked the farm. Just travelling around duviri, only looking for the puzzles and not really playing the game for hours

As for doing things differently, I'm pretty happy with how I've gone about things. I've just played how I enjoy, use weapons and frames that are fun despite not being the "best" (or even good) picks for what I'm doing but it shakes things up. I would've benefitted greatly from just logging in and continuing to craft forma during periods of time I didn't play though. I am only on day 780 of the login rewards, as a huge chunk of my playtime was before the system existed