Have you tried the lower heat method? From what I have read about preventing crystalization when you don't want to use the lemon juice and alum then the best method is to simmer only and stir constantly when adding the sugar rather than adding it to boiling liquid.

"Simmering the syrup for 10 minutes, instead of merely bringing it to a boil, inverted enough of the sugar without affecting flavor."

Here is a link to the article I'm referencing. Hope it helps.

Dave

https://www.cooksillustrated.com/how_tos/8617-preventing-sugar-crystals-in-simple-syrup

I'm just wondering if it crystallizes over time. Maple syrup does, but so what if there is some rock candy at the bottom of the jar. How much simple syrup does one woman need! I'm just wondering if anyone has any experience with this. Odd that there is not much about this on the Web. Most say to store it in the fridge, and I may just do that, I'm not going to make batches and batches of it. Or I may get my freezer up and running here soon. I want to make lemon balm, cinnamon basil, lemon basil and maybe thyme and ginger syrup, some combination. I don't want to add lemon juice because it would overpower the delicate flavor of the herbs. I have made herbal jelly, and even apple juice is a little too much additional flavoring. I suppose I could make something like cinnamon basil peach ginger syrup, but I dunno, I wanna keep it simple. The only acidic thing that is in season here now are late season raspberries. I guess some blueberries too, but they are not acidic.