I've made grape, blackberry, elderberry, pear and apple jams and jellies, as well as combinations of those fruits, for years with Sure Jell dry pectin. I sometimes stir in spices like cinnamon for grapes and blackberries, nutmeg for pears and apples, and even lavender flowers or mint. Elderberries grow wild in our area. I use the instruction sheet in the box. Occasionally, some of the jam or jelly turned out to have a gritty or grainy texture. This has happened more in the last couple of years.
As noted, it would seem to be important to use fresh pectin (watch the expiration date on the box), stir it very thoroughly into cold or room temp. juice before cooking, and, yes, be sure sugar is all dissolved by turning down heat and stirring a lot before boiling juice. I'm going to watch this more carefully.
Also, if possible, pick your fruit the same day you make the product, especially grapes. If you cook down grapes and let the juice sit in the fridge overnight before you make the jelly or jam, it will form gritty oxalic crystals that will definitely make your jam or jelly crunchy.
If you're making a product with apples or elderberries, know that they are naturally rich in pectin and will jell quicker and with less Sure Jell.
Good luck! I sell my jams and jellies at the fall church festival and make money for the church.