Is trading volume important in buy and hold ETF strategy ?

The bid-ask is marginal now is because the stock market is not as volatile as compared during a crisis.

Volatility increases during periods of rapid market decline, usually from a financial crisis. At these times, the bid-ask spread is much wider because market makers want to take advantage of and profit from it. When volatility is low, and uncertainty and risk are at a minimum, the bid-ask spread is narrow.

Trading volume is important upon selling only, especially on a critical period. If you are selling in the mid of crisis and there are no buyers to match your selling price, you might not get out of it in time as well. Even so, the spread might be huge with poor supply and demand if you choose to sell with market price as mentioned earlier on.

Therefore, chances are be prepared to hold forever and have the tolerance to tide through any crisis at any life stages for the best. But no doubt, we can't predict what will happen the next 30 years or more, but just a certain scenario for you to consider.

I personally find AUM being a more important indicator.