So I am thinking, apart from physically confiscating hardware wallets and apprehending bitcoin holders to submit their seed phrase, is it possible that the government can seize bitcoins belonging to citizens even though it is a decentralized system?
No they can't and that's what give Bitcoin power as a decentralized currency, the government missed their chances of seizing or buying most of the Bitcoin in circulation as of when Bitcoin was newly launched and started making waves for itself. Back then they could have easily gotten as much Bitcoin for a very low price and continue buying more of the mined Bitcoin coming into circulation but they doubted Bitcoin by thinking it won't be much of a threat to them. They didn't believe Bitcoin would come this far and now it's already too late for them to achieve seizing or buying most of the Bitcoin in circulation. Using the word most because they can't fully have control over all the Bitcoin in circulation. Lets not forget new coins are been mined daily and introduce into the market and even though the government takes over mining, we'll still have independent miners.
So far as the fund is passing through CEX, it is possible to seize them. To cash out cryptocurrency, you need KYC through which the identity of the user is known. It is just a matter of running background check on the user to check their activities and affiliations.
A decent amount of money is been traded on decentralized exchange so not everyone uses centralized exchange and if the government through CEX keep up their recent activity of seizing funds on centralized exchange, it'll make others prefer to use DEX instead of CEX. Those exchange seizing customer coins are building a bad reputation among the community as this is a decentralized community that's supposed to be exempted from the regulations of the government. Alot of DEX are becoming more popular and we might just start seeing high volume as more investors pick them over CEX. During the defi hype we saw DEx competing with CEX for volume and we can't repeat similar scenarios again