My turn........
I have a bunch of the PC 690-style routers - that is what you have. SOme old fixed speed 1.5 HP, some newer VS 1.75 HP. Would not bother me to use them in this application - - - the downside is that you might not be able to do it in one pass. A bigger HP router has its advantages, but you also realize its disadvantages. I have no problem with the 690 - I think you are overdriving it currently.
I have the impression that the PC 8xx series is now officially junk - get the DW instead, if you go to this class starcruiser. The PC 75xx is still good, but man - I would not want to be wraslin' that horse around - it is best suited to a router table, IMO.
Or - go full-on D-Day on the problem, and get a used overhead pin router.......
Reading your process description, I am all but certain that your template sits on top of the 1/2" ply, and the "template guide" you refer to is the brass templet bushing, correct? Just double-checking on important detail.
Two options:
Multiple passes:
Template on top, brass templet bushing installed in the base plate; Use a 1/4" cutter diameter, on a plunge base. Make 3 pases at about 3/16" each. If you can feel/hear the motor bogging down, you are pushing too hard/fast.
Zip then route:
Do a 2-step process. Don't know what you used as a guide/template when you used the Rotozip?
But - the way to do this method is to use the rotozip and cut out the waste - but stay off the final line. Then closer you can get to the line, the better - but stay just off of it.
Use the template and the router - but with a flush-trim bit instead of the spiral. You are taking off much less waste in total, plus you are only cutting on one side of the bit - much less load on the router.
You also have the option of a "plain" flush trim, or a top-bearing flush trim that runs against the template without your brass bushings installed. You can even us a bottom-bearing bit, and put the template
underneath the 1/2" ply.