Buy alts when you see btc standing stable in sideways because that gives an opportunity for the whales to divert their money in alts and that is when casual traders like you and me can make money. The best part of altcoin trading is, they must be traded in USDT and not against btc because if an alt falls against btc and btc too falls in price, then you are at higher loss compared to what you will lose when you check the USDT price while buying that alt.
If you are going to go far a short-term investment, it seems very risky in terms of not only Bitcoin but also even the top altcoins in the market like Ethereum, BNB etc.. Because the market has not been in a good shape in the recent times. China's action also has an affect on the price I think as it is like an obstacle for the price to climb. If you are aiming for a long-term one, then go for it. Otherwise, you'd better observe the market for some time.