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I would sell half of my BTC for USD/USDT and wouldn't touch it for now, with the other half you can buy 50% some quality altcoins which you believe in and keep the other 50% in BTC and see what happens and take your chance as bullrun most probably isn't even close to over yet, but you should always take some profits while you can so that you don't get out of your trade/investment with a loss at least.
If you want to buy altcoins, be fully diversified into 4-5 different projects with different use cases, for example do not buy/invest in 5 DeFi projects and think you're safe now, since usually every new hype whether it's DeFi, NFT or whatever else has to correct at some point and you don't want to be all in one of them when/if that happens.
Half is quite big already to be honest if one sold that out of the whole portfolio. It would depend if the OP still has other investment aside from Bitcoin, but if it is only Bitcoin I wouldn't sell by half maybe in alts I would because dumps and losing value are quite common phenomenon when it comes to following BTC price.
Depending on how much OP has in his/her capital, the 4-5 investment could be quite big already.