Did you also check that the door interlocks are working properly?
Here’s an image of the magnetron circuit for a G.E. microwave.
It is NOT for your model but may be close enough to be of some help as it shows how the door interlock switch if not opening when the door is shut will still short out the HV transformer preventing current flowing through it etc.
If the interlock is OK then you may have to use an Ohmmeter to check the circuit supplying the HV section of the oven, if everything else seems to be operating OK, i.e. turntable motor, light, fan etc but there is just no heat.
It looks like the antenna dome of the magntron has burnt out.
Here's a link that describes it better than I can.
You said that the magnetron tested OK. Is this the method that you used to test it?
Even if it tests OK with an Ohmmeter it is not being subjected to the same amount of voltage with an Ohmmeter that is being applied when operating.
To do the 2nd method, unless there is a short circuit or low resistance path between the housing and a terminal you really need a megger tester (example only) as this will test if the insulation breaks down at the voltages that the equipment operates at and not the 9V of an Ohmmeter.
How old is the oven?
Looking at the owner manual there is a 10 year warranty on the magnetron. Not as good as it sounds as yr 1 is full parts and labour warranty and yrs 2-10 is part only ,you pay for the service call and labour.
Although it may be worth considering if applicable and you can install it yourself given the cost of a replacement.
If the warranty period is no longer applicable or even if it is and the manufacturer will only supply and install and not just supply only, then it may be more economically viable to get a new microwave oven.