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

First of all, it all depends on what you actually want as a person, like are you a long term investor or a trader?
If you are actually a long term investor in Bitcoin, then I would advise that their is no better time to buy Bitcoin than now.
What you just have to do is to utilize the DCA accumulating strategy, which is buying Bitcoin bit by bit either weekly or monthly regardless of it present price, and before you will know it, you may have accumulated a very good stash of Bitcoin, because the stash of Bitcoin in your possession determine how profitable you might be in the future, so since Bitcoin is an asset that appreciate in value overtime and with it huge potential it might skyrocket to a million dollar in like 10 to 20 years time from now, so the DCA method is the best if you are holding for long term purpose.
But if you are a trader that buy and sells at any slight increase in the price of Bitcoin, then you can utilize this approach you are talking of, but take note that this kind of approach wouldn't have much impact on your financial status forever, unlike when you buy and hold for a very long period of time, but it all depends on you and what you want though.