Yes agree not all coins are affected by Bitcoin but most of the major coins are affected for every Bitcoin price action that is why there is a chart in tradingview related to BTC.D meaning dominance as you can see it is currently around 50% above meaning it still dominates most of the altcoins including memes.
The impact of Bitcoin on the altcoin market is of course there, yes. Mostly even in such a way that the alts reflect the price reaction of Bitcoin more clearly, e.g. falling or rising more strongly. However, this has hardly any effect on trading, as the time windows in which one trades are usually so short that neither the general market sentiment nor the current Bitcoin dominance (can) have an influence.
That is why when you trade always analyze the BTC price action first because it can increase your validation and probability that the position you just placed has a high chance of winning that is why there is a bias included in the list above to increase your chance to win a trade.
What trades are you talking about here? On short time frames (minutes, hours up to a day) you don't need to analyze or consider this, as the time frame is far too short and other factors (e.g. trading volume) have a much greater impact.
For longer-term trades (days, weeks, months) it is of course different, but the respective market sentiment (bullish vs. bearish) plays a much more important role.