Owner, you are obviously conscientious and very self-aware. You know that you come across as gruff to the general public. It sounds like you like the sport, but worry heavily about liability. Based on your answers, I assume a full-time RSO is out of the question. These are my assumptions preceding my posting of answers.
First, you must get comfortable with the risk your business has. A review of your insurance with your agent as well as a trip to your attorney to update things will help. The insurance agent should approve your safety program and the attorney should make sure your business can in no way make you personally liable for a catastrophic accident.
Second, pre-selling or pre-conditioning the public goes a long way to both informing them and lessening your "abrasive" contact with them. Very concrete rules should be generated and posted on signs both inside and outside of your store and range, at the counter and on your website. Rules for range use should never be subjective, I.e. subject to the way someone holds a gun. Some may have disabilities or genetics that make their grip look strange. An objective rule would be "Both hands must be on the gun when fired." You could even provide a picture chart of acceptable stances and grips, with big "x"s over unacceptable examples. People learn far better from passive examples than active criticism. Quite frankly, bloody fingers are a great educator, just keep band-aids around! No one is going to sue you over a cracked knuckle.
A new indoor modern range near me uses a 4-minute video that must be viewed before there is any range use the first time. In the video, safety rules are reviewed and penalties for not following same are hammered home. Beginning shooters are told to ask any personnel or the RSO if they have ANY questions. Any shooting organization, such as NRA, should be able to provide teaching materials.
Third, If beginning shooters are your biggest worry, have two days a week, Sunday and Wednesday or your two slowest, that are "New Shooter Class" day, another sign that will adorn the shop, range and counter. Provide separate business cards that current members can pass out with class details, so a new shooter understands their first visit MUST be one of those days. Provide a friendly, personable RSO that can teach on those days.
If you cannot afford a full- or even part-time RSO, trade range time with one or two customers who can be friendly, courteous, and are able to train with patience. You have nothing out-of-pocket, but you provide RSOs better able to handle newbies than you are. They only need to work on new shooter days, so it shouldn't be hard to find people willing to help.
Finally, educate your current customers. Explain new shooters may only come on designated days. Tell them what you consider "unsafe" behavior and ask them to report it if they see it. Definitely be firm in your enforcement, but temper it with politeness, much like police do. You are in a selling business to the public, and everyone is offended by something! It is your range, and you will always decide who uses it and who doesn't - the difference is that with signs, education campaigns, RSOs to take the pressure off of you and objective rules that can be quantified, everyone knows EXACTLY where you stand and why, and there is less chance of a negative encounter. You already have customers referring new shooters, so you must be doing something right! You just need to manage it so customers understand everything before they even get to the counter.