It's difficult on that way, not all the time Ethereum will pump and Bitcoin will do sideways, sometime if Bitcoin is dumping altcoins are dumping too, especially Ethereum and some large market cap altcoins.
The best way is to have both technical analysis for a pair you want to trade, seperate with Bitcoin and Ethereum for example. Sometime Bitcoin and Ethereum correlate, but the best to do is to have analysis first.

If the market is sufficiently liquid for your needs (i.e. you won't be constantly experiencing bad slippage) then it really doesn't matter whether you trade against BTC or USDT.

I only trade against BTC personally because my goal is to generate BTC profits, not USD profits. Trading against and caring about fiat value would only distract from that. If altcoin trading is losing me BTC, that's a huge fail.

It also makes more sense to trade ALT/BTC markets because you basically never want to be holding fiat in this kind of bull market.

On the other hand, if you're the type of person who is very scared of holding crypto, and more concerned about preserving USD value than BTC value, then trading against USDT probably makes more sense.

Just don't get greedy and think it's going to last forever. If you start hearing "flippening" talk, you should probably be taking profits off the table.