If a quick, limited strike was the plan, it's better to stick to the plan. There were multiple variables involved. They couldn't be 100% sure Hinata would be able to slay Rimuru. They wouldn't actually know with absolute certainty there would be no disastrous surprises they could never imagine waiting for them if they spent too much time in the city with the limited group of knights and the three summoned lunatics. This all in addition to what Ryll already said.