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

this is entirely up to you, a lot of people can recommend where you should put your profit but at the end of the day, you should do what you are comfortable with.
Put 100% into bitcoin. Everything else is garbage.
this could work if OP isn't looking for a variety of crypto to invest in. but if you ask me it's probably one of the best thing the OP could do.