Try this. Get a half inch grommet which will be about an eighth of an inch thick, put it in the collet first, then bottom the the bit on the grommet. That might give you just enough reach to finish the cut. Are you trying to cut all the way through your base? Is the base single or double thickness. I always prefer double, ply on top, MDF below. That gives you an inch and a half total thickness, and you don't want the whole opening to be that depth, only about half an inch. With most mounting plates, that gives you 3/16ths of height adjustment space, which is the job of the height adjusters.
That means you want to cut out an opening half an inch all around, inside the outline of the plate. That gives you a shelf to keep the plate from falling through. Sounds like you are still trying to make the cut with the same setup. A bit like trying to pound a nail with screwdriver. With a 3/4 inch mortising bit and bearing, AND NO ADAPTERS OR OTHER ACCESSORIES, you should be able to make the "shelf" I've described.
Cut the smaller opening with a jig saw, then cut the shelf using the template. You may be able to find the mortising bit at a big box store, or order it from Freud on Amazon or at Home Depot online. I think you have over-complicated this project. Theoretically, you could make the cut the way you're trying, but it will be very difficult to get it right. The bushing offsets the straight bit so it will cut inside the line of the pattern, and the adapters restrict how far you can reach with that kind of bit. A double whammy so to speak.
Many of us have made or adapted our router tables using the mortising type bit only. A straight bit with bearing is almost the same as a mortising bit, but doesn't flatten the bottom quite as well. A straight bit without a bearing and trying to use a bushing, will give you the fits you're experiencing.
You only want to make that shelf (a rabbet actually) slightly deeper than the thickness of the plate. The height adjusting gadgets will let you raise the plate even with the top. Below is a rough diagram (cross section) of what I'm talking about. The big arrow represents the height adjustment screw.
Here's a little video of someone doing this. Note that he is NOT wearing a mask-bad bad bad. But he is making the rabbit and using simple lag bolts through the four corners of the rabbit, on which the plate sits.