FDev, you are NEVER, EVER going to "balance" anything with regard to Power Play while you continue to allow it be impacted by players in Solo and Private Group! Open and Solo / Private Group are two diametrically opposed paradigms that can NOT be reconciled under any circumstance, no matter how hard you try to force that square peg into the round hole.
Power Play in Open mode carries the risk of being countered by other real players; it has its own built-in throttle to prevent run-away Control Score gain for Galactic Powers, assuming players avail themselves of it.
Solo and Private Group, by contrast, have no such limiter. There is no risk, there is no consequence, there is no counter. Galactic Powers' Control Scores can sore through the roof overnight as Solo and Private Group players dump Fleet Carriers'-worth of rare goods, billions in exploration and exobiology data, farm SLFs at Stronghold carriers without fear of real player intervention, etc. Which appears to be what you're taking issue with; the too-rapid a swing in Control Scores and an upending of the grand strategy game that you had hoped Power Play would become.
And your knee-jerk response? Nerf every Merit-generating method available, stubbornly clinging to the belief that you can salvage the situation, that you can make Power Play all-inclusive across all modes. Ummm...<psst>...you CAN'T ! Not from lack of trying (because God knows you've tried; you've kicked that dead horse until it's turned into an unrecognizable red smear).
Nerfing Merit-gain in service to the all-mighty mantra of "BALANCE" is precisely the wrong solution. You're attempting to balance that which can not be balanced because there are fundamentally two different games under one umbrella at work here; a high risk / high reward version of Elite Dangerous with hard counters (Open mode) and a no risk / all reward version of Elite Dangerous with no counters (Solo and Private Group). But rather than balance, all you've accomplished is to alienate everyone; Open, Solo, and Private Group moders alike. You're rapidly eroding all the good will you've built up over this past year and players are not going to be kind to you (see Helldivers 2).
The solution is to finally, FINALLY, accept that you can't have everyone play the same game when you, yourselves, created two different games under one banner. You created the problem by trying to shoehorn two incompatible modes into one game. THAT can not be undone; it is what it is, now. But, with respect to this most recent debacle of Power Play, it's salvageable and here's how:
Merits earned in Solo and Private Group will no longer contribute to the Control Score of Galactic Powers.
Merits are still earned. They're even still earned at the same rate as a Commander playing in Open (no worries of cries of "UNFAIR!" due to a disparity in Merits earned between modes). Players in Solo and Private Group continue to progress through their ranks, unlocking modules and Galactic Power-specific bonuses just like their Open mode counterparts. This group is happy; they're getting what they want out of Power Play (rank progression, modules, and no perceived inequity in Merit-earning potential).
Meanwhile, those playing in Open get what they want; a Power Play wherein counters exist, where no longer can a Galactic Power's fortunes be influenced by Commanders in Solo and Private Group who are de facto playing the game in Invulnerable + Stealth mode. Power Play is returned to a strategy game conducted on a level playing field where the risk / reward paradigm is reinstated and respected. This group is happy.
With no Merits contributing to Control Score in Solo and Private Group and hard counters existing in Open, Power Play settles down to a more measured pace in alignment with FDev's vision for this facet of the game. This group, too, is happy.
The only group that won't be happy is the one who willfully uses Solo and Private Group to engage with Power Play consequence-free, knowing they can shift the fortunes of a Galactic Power and there is nothing anyone can do to counter them. But, as that approach is the antithesis of what makes for a good strategy game, I don't think the other three groups will care if this minority is unhappy that their no risk / all reward play has been eliminated.
So we score a WIN-WIN-WIN for the three groups that matter with the fourth's opinion being irrelevant.
And, to address the loophole that some readers are probably already contemplating, anything that generates Merits will be "tagged" from this point forward. If it's obtained in Solo or Private Group, it will be identified as such, even if only behind the scenes. This Solo / Private Group tag is permanent and can not be reversed or changed; it's a Merit-generator that will remain tagged as having come from Solo or Private Group in perpetuity. If a Commander in possession of a Merit-generator tagged as such enters Open, its Solo / Private Group tag remains. It can be redeemed for Merits, but will not contribute to a change in Control Score under any circumstance.
If, on the other hand, a Merit-generator is obtained in Open it, too, will be tagged as such. Unlike the Solo and Private Group tag, though, it can change tag. If an "Open" tagged Merit-generator is brought into Solo / Private Group, it becomes a Solo / Private Group-tagged Merit-generator. The reason for this is to prevent Commanders from gaming the system. Some examples to illustrate:
A Commander, playing in Solo mode, returns from a lengthy exploration journey with a billion credits in cartographic data. They safely dock at a starport in the Bubble, never having worried about being interdicted by real players. They log out to the Main Menu and then back in again in Open mode this time. Their intent being to sell their exploration data to contribute to their Galactic Power's Control Score in the system, patting themselves on the back for how clever they are. But, because the cartographic data was obtained in Solo mode and is permanently tagged as such, the sale - though earning Merits for the Commander - has no impact on the Galactic Power's Control Score.
Another Commander, this time playing in Open mode, docks at a starport and buys out all the local rare goods. They then switch to Solo mode and fly to their desired destination, using this mode to avoid interdiction by real players. Once safely docked at their target starport, they switch back to Open, believing the sale of the rare goods will not only net them Merits but contribute to their Galactic Power's Control Score, too. Au contraire! The moment they changed to Solo from Open mode, their rare goods also changed tag from "Open" to "Solo"; a permanent change. From that point forward those rare goods will NEVER be able to earn Control Score for a Galactic Power - Merits for the Commander, yes, but Control Score for the GP, no - even if the Commander in possession of them returns to Open mode.
In this way Commanders are prevented from gaming the system. Control Score can only be affected by those Commanders who play in Open and remain in Open while conducting Power Play activities. Unlike many previous calls for "Power Play should be Open-only!", though, it preserves the ability of Solo and Private Group players to still pursue the Merit-gain, rank progression, module unlocks, and bonus acquisition of Power Play. The only thing they're unable to do - while they remain in Solo and Private group or bring a Solo / Private Group-tagged Merit-generator into Open - is impact the Control Scores of Galactic Powers; as it should be.
Then we can finally dispense with the folly that is attempting to balance two incompatible game modes around Power Play; all while keeping nearly everyone happy (save those who were essentially using Solo / Private group as a de facto Power Play exploit).
