I have a KGSA906PSS00 which is also a superba oven and it is 10+ years old. I didn't notice a model number in your post....Over thanksgiving my oven stopped heating properly at all. I have waited until recently to address the issue and tried the self clean option the other weekend to further troubleshoot the heating issue. It did lock and heat up to a high enough temperature and blew the thermal fuse. Good and bad - as now the oven was locked and powered down until fixed. However, it did alert me that the issue was the thermostat with regards to the actual (lack) of oven heat. I ordered a new thermal fuse and a new thermostat and actually repaired last night. I will say that both are a very easy repair; however, the thermostat is especially easy when you have the back cover off to better access clip. This is the 2nd time I have replaced the thermal fuse and the 1st time I have replaced the thermostat.

I hope this helps you troubleshoot your issue.

your oven looks similar to mine; likely a newer model than mine. Again, I'd suggest a new thermostat which you can find part number on this website or your parts manual in the "oven parts" section of the pictures on the appliance parts pros website. Likely, if you shopped around you could find a less expensive new one ($20). Let's just say those 2 items I replaced above with new parts costing less than a total of $40 by shopping around.

Also, if you did a general search or youtube search on troubleshooting oven not heating up it will likely come back as thermostat. I'd suggest you do this too. Do a search.
Again, I hope this helped you troubleshoot your problem.

220 was the temperature it reached when I set to convention bake at 425 but it took 30mins to get to that temperature of 220

As mentioned if I use the non convention bake it will heat up as normal but then that takes about twice as long to cool things as convention

Any advice would be greatly appreciated!