you are getting a lot of things wrong here!
as I can assess, right now, bitcoin is at its high turning point (11k USD),
$11k is not a "turning point" it is simply a resistance. and there is no reason for a drop just because you think a price level is a resistance.
as well as some other altcoins I am holding.
majority of altcoins don't have any resistance, they just have ATHs that are reached from their pumps and then downward trend they start right after their dumps begin. which means you shouldn't be holding them in first place.
I would like to find a currency that prevents this loss from fluctuation.
currencies don't do that, least of which cryptocurrencies and definitely not altcoins.
if you want least amount of volatility then your choice is fiat although fiat can also tank heavily.
Tether f.e. (handled in BTC) fluctuates exactly the same as bitcoin in USD.
no it does not. Tether is called a "stablecoin" for a reason! although it should not be used since it is a centralized and extremely risky coin.
Which altcoin would you advise for short / long term stability?
i've already explained why you shouldn't hold any altcoins. here is another reason, whenever bitcoin rises or falls altcoins get dumped. so even if you are worried about fluctuations, you shouldn't be holding altcoins. hold bitcoin or fiat.