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

To me is better to start now with some DCAing, and you can start with any amount of your choice there's no deadlines or something, so there's no need to be in rush , just start accumulating and keep your investment smooth, but don't over do it when purchasing just in the name of trying to be aggressive ( like using your emergency funds to purchase). So is for you to decide the amount you wanna use to start the only advice I can give to you for now is to start with DCAing.