Set response-time expectations on day one
Most client frustration during onboarding comes from not knowing how slow is normal. A client who expects an answer within the hour and waits a full day feels ignored, even if a day is completely reasonable given how you actually work.
Fix this by saying the quiet part out loud, early in the relationship. I say mine outright in my very first message now, and it has cut down on plenty of anxious follow ups. Tell new clients plainly how quickly you typically respond and during what hours. This single sentence prevents a huge share of the anxious follow-up messages that would otherwise show up in your inbox uninvited.
Setting this expectation also protects you. Once it's stated clearly, a client who reaches out again after only a short wait is much easier to gently remind of what you already told them upfront.
Try this today: add one clear sentence about your typical response time to your welcome message, and start using it with every new client from now on.