Hello there. Heating elements have always been hard to get out after they've been in for a long time because of the way they are held in place by the metal plate squashing the rubber. The longer one is left in place the more the rubber takes on the squashed shape and becomes very difficult to pull out.
Back in the day all of the heating elements were fitted into aluminium back plates. They were still hard to get out but at least you had a solid metal surround which would be very difficult to damage. Difficult elements were prised out with large flat bladed screwdrivers. These days they are nearly always slotted into plastic tubs and they can easily be damaged.
If you've loosened the central nut and knocked the bolt shaft in to move the plate at the back away from the rubber seal you can normally carefully prise out a heating element, but if it is not coming out then the only technique I've found to work is to undo the central nut to a point where it is almost off the thread then in the shaft of the bolt so that the nut is flush with metal on the heating element. This should force the metal plate inside far away from the rubber.
The only thing holding it in place now is the squashed rubber. If it won't come out you have to ease it out a bit at a time all the way around and I often found a small screwdriver, and even occasionally some narrow nosed pliers can pull the rubber up a bit at a time. Sometimes I found a remarkably small flat bladed screwdriver could just get in and if pushed in firmly and lifted upwards would just ease a small bit of the rubber forward. If repeated at both ends and all the way around you can slowly work the rubber out. After a while narrow nosed pliers can sometimes grip the rubber and pull it forward a bit more.
I occasionally also used a large flat bladed screwdriver placed close to the heating element so that I could then use another screwdriver resting on it to create leverage. This meant that the main force of the leverage was against the shaft of the larger screwdriver instead of against the plastic surround of the heating element which could easily break.