Hi im trading altcoins on exchanges for years now and i was always withdrawing like 60-70 percent of my profit
and buy bitcoin for the rest (30-40 percent) but since btc price is highly increased you think i should just keep
doing the same or i should buy USDT instead with that 30-40 percent meant for bitcoin and store it in a safe wallet
like trezor till btc price drop or till we see a new bear market then buy btc with that saved USDT

I guess you are doing the right thing by using DCA method to accumulate BTC in your wallet and you will have a good reward when you are ready to sell them in the bull run in the next 10 years because by then the price of BTC will pump above $1m to allow hodlers to take the advantage of the price to make massive profits. Even though you want to add another coin to the BTC, let it be decentralize currency because they are reliable more than those centralize currency that don't use to help customers to grow profits faster in the long run.
I believe you are using electrum wallet to hodl your BTC? If no, please try and download electrum wallet to hodl your BTC because the wallet is well secure and safe for long hodling, and it will be very hard for scammers to have access to your BTC in your electrum wallet.