Why not use pectin, the default gelling agent for such things? It's about as natural as you can get.
'gelatine the normal jelly thickener.
can do, but it does have a different mouth feel.
why are you against gelatine, imo the king of gelling agents. unless you want something unique.
like agar agar only melts at something like 80 degrees and you have to be very exact with measurements.
Indeed, it reduces the water activity.sugar might be better to leave in (so it doesn't go bad as soon)