The extra step of pouring syrup is simple if you use a syrup hand pump. You can get them at restaurant supply stores and usually it'd be a pump or 2 per glass. That should make it easier for the lazy people when your not there . As for the recipe, you don't really need to pasteurize the lemon juice, just the strawberry juice. I'd say boil the 4 cups of strawberry juice with the 6 cups of sugar for 45 seconds, then add the fresh lemon juice and citric acid. Just make sure you scrub the lemons with the soft side of a sponge (don't use your usual dish sponge either, you don't want to know how much bacteria is in one of those once they've sat out for a couple days) and sterilize your hands and juicer before extracting the juice. For sterilizing I usually use a spray bottle with rubbing alcohol and water, works great, but my wife usually makes me spray some Febreze afterwards.
A flavor change in the strawberry juice should only have occurred if it was boiled too long, I think the jam flavor might have came from the additional sugar and reduced juice if anything. Your previous recipe had 2 1/2 cups of juice per 3l of finished soda, your new recipe is 6 cups per 23l (6 gallons) of finished product. Based on your original recipe you would have needed ~19 cups of strawberry juice to keep the flavor the same, 13 more than you used. You could try adding 12oz containers of frozen strawberry juice concentrate, they are the equivalent of about 6 more cups of juice each and might make up the missing juice flavor. Because your making such a big batch you could pour some off to testing glasses and experiment with it till you find the mix you want. From my experience even a simple lime soda took several tries before it was perfect.
A "Mom and Tot" special on the menu could boost sales. A sparkling strawberry wine or other strawberry based alcoholic beverage for the parent and a strawberry soda for the child. You could also make it seasonal; a pumpkin ale for winter, sparkling lemon-aide or "candied" orange-aide for summer, etc. Those can also be made into sodas or alcoholic drinks as well.