The Dread Slender Gnome on Nostr: The jam I made yesterday out of the kind-a-damson fruit came out so good, it should ...

The jam I made yesterday out of the kind-a-damson fruit came out so good, it should be illegal.

Used the River Cottage book again, namely it's recipe for plum jam's orange juice+cinnamon variation, with added angostura and cardamom.

And it settled all right too, into a soft, spoonable jam.
The only confusing thing was that I could not get the temp of the jam to rise to 104,5C, which the book gives as the setting point. My thermometer just about got to 96C.
I boiled the jam the recommended time and a bit extra, but then stopped because the consistency clearly changed and I was worried about it boiling over (yes, too small a pot/pan, I know).
My thermometer was one of those digital ones with a probe at the end of a cord. Do those not work for jams? Do I need an actual jam thermometer? Or was the problem having a fairly wide and shallow pot, as opposed to a regular one?
Or was it because I run out of sugar (!) and had slightly less than the recipe called for, I think 100g or so?

Either way, the jam came out perfectly acceptable in consistency, so clearly the temperature issue wasn't a deal breaker. The jam is absolutely delicious and I'll be eating it with everything I can think of (within reason).