I disagree that it would be a mistake. Even Tinto themselves seem to think that opening up previously Tag locked mechanics is a good idea. During the EU4 government reform rework, they opened up thing like Sortition, Merchant Republics, Parliament, Monarchists vs Statists, etc. And to my mind all of that was a net good for the game. It improved replayability and enjoyment by letting other Tags get access to things that used to be locked. I just want to go a step farther and open up more of those unique things. The fact that Zulu also has access to "Prussia's" Mechanics already indicates that there's nothing inherently Prussian about them.
As we are using the EU4 Prussian Government mechanics as our example here, I'll continue to do so. I'd generalize it (in EU4 terms) to make the requirements be something like "Have Quality Ideas and one of Offensive/Defensive", by taking those ideas you have already taken steps to go "all in" on the military, and then the government reform would be a continuation and formal entrenchment of that path. After all, a small nation like Tibet or Nepal trapped between China and India, might in start to fear for their safety and go down a highly militarized path. Would it look exactly like Prussia's path IRL? Probably not, but the mechanics in the game are already an abstraction available to "Not Prussia" so for me there is only benefit to opening them up even more just like they've already done with other previously tag locked mechanics.
With all of that said, who even knows what sort of thing might represent Prussian militarism in PC, or honestly even if anything will. But for me, the fun of Paradox games is the "what if" where some random nation can make it big against the odds or develop in a way that's totally different to reality. Colonial Mamluks colonizing Australia, or the occasional Mega-Albania comes to mind. The problems with CK3 (or Victoria 3) are not that all the mechanics are available everywhere. That's a misdiagnosis, repeated because people have EU4/HOI4 mission tree brain "Those games are good/fun and they have mission trees/unique mechanics, therefore mission trees/unique mechanics make a game good". CK3 has problems because it's so easy to become overpowered by accident, even when you are actively trying to avoid it. Victoria 3's issue is that the existing mechanics don't even work correctly, and even when they do the game itself is just kind of dull, you spend too much time waiting for things to happen. CK2 didn't have tag specific mechanics, a count in France played exactly the same as a count in Italy after all, and most folks here would probably say that game was quite good and very replayable. For me, EU4 isn't good because of the tag specific mechanics, it's good in spite of them. It would be better if Prussian militarism was more widely available, or if more nations could become a "holy horde", or if more than just Egypt had access to the "Egyptian Westernization" mechanic, or if more than just the Mughals had Diwan. Good mechanics and fun gameplay make a game good, not "Tag Magic". And good mechanics are mechanics that you can actually use so the more accessible the better.