That the current discussion is dominated by module shoppers and rank grinders is unfortunate and possibly detrimental to PP, in my opinion.
You can ignore all that and still find come to the same root cause of merit rewards being unbalanced across activities and unlocks taking more effort than before.
It would have been great if the thargoid war had this level of info on how much certain activities help with progress, the flaws with orhtrus hunting in alerts and sampling vs combat being imbalanced early on would've been more obvious and easier to catch. The in-game info is still pretty bad at telling you how much you'll get for doing something before you do it which leads to requiring players to experiment in weird ways that could disrupt normal gameplay if you don't want to just look it up online. If powerplay is meant to be at all strategic then encouraging a more planning-focused playstyle even on the individual level would be a better fit than experimenting thats just throwing crap at the wall until you find the good activity for getting merits and/or helping your power.
I abused the hell out of the relic trade and got rank 729k merits (rank 94) the first week so I could ignore the progression system. I wish I could say there's a good progression system hidden behind all the grind, but there isn't. There's definitely more fun ways to do progression systems and unlocks than this (or most other unlocks in Elite).
Unless some other power has insane bonuses to PP cargo trading that I'm missing I think just swapping the rewards for rares trading and delivering PP cargo would be a good fix. Whatever rebalance there is it'll probably feel like pulling up the ladder similar to players who missed the exobiology elite rank after the scanning rewards were rebalaned. Unless the merit rewards are also multiplied by a significant amount, which is how a lot of other credit reward balances ended up.