BTC Only don’t touch alts
There is nothing wrong in altcoins. Why shouldn't you invest in them? in 2020-2021 you could generate a huge profit by investing money in Matic polygon and it was not a shit memecoin, it was a valid coin. BNB was a good option too because that halving was the time of rise for Binance as a trading platform. There are many details that you have to consider because it's not 2016 today when you could 100x your capital in some months via holding bitcoin only.
using BTC as collateral to borrow USD for Altcoin investments.
Nice plan exchanger like Binance offer collateral loan with low interest. so we can invest our money on bitcoin and take some loan by collateral our BTC then invest in Altcoins. Altcoin is much profitable and when bitcoin pump altcoins also pump with Bitcoin. So if the market is good then in this case we will get several times more profit from both investments
If you want short term trading with altcoins, you can move your bitcoins into futures and open short or long position while your asset will be calculated in bitcoins. Pros of this method is that if you open short and bitcoin price goes high, in case your short position isn't doing well, your bitcoin's rise will compensate it but cons is that if you open long position but its not going well and bitcoin's price is going down, you'll accelerate go to the way of liquidation.