Some governments (like El Salvador and Russia) are moving into bitcoin mining.
If more nation-states accumulate BTC, what impact will this have on decentralization and price stability?
If more nation-states accumulate BTC their impact on:
Decentralization:
Bitcoin network is fully decentralized, I do not think the government can be able to make the Bitcoin network centralized, they can only integrate centralized application on the Bitcoin network but can never affect the blockchain to make it centralized. So I believe there will be no impact on the decentralization aspect of the Bitcoin network. Holding Bitcoin which is POW, is very different from holding a cryptocurrency that has a staking feature like ETH.
If we talk about the market, obviously those who have control over huge amount of Bitcoin can always manipulate the market price, it may be considered to reduce the decentralization of Bitcoin since some powers are obstructing the decentralized market of Bitcoin, but if we talk about the Bitcoin network activity, still I do not think the decentralization will be affected.
About the price stability,
If the nation decides Bitcoin to be their reserve, then we can see a long-term stability of Bitcoin, but if the country just wanted to play the market, then the price stability will be affected due to buy and sell action of a huge holder especially if they take advantage of the market uptrend, in short, it will make the Bitcoin price highly volatile.
Could this lead to geopolitical bitcoin wars?
There is a possibility between nation competing to amass Bitcoin, and or if there is a conflict between country and one tried to avoid the sanction by using Bitcoin. As long as there is greed, the possibility of geopolitical wars is always there, may it be Bitcoin or other things.