I may not have fully understood what you were saying.
What you have drawn is what I had in mind, but I understood that you wanted to keep the fine adjustment feature. To do what you have illustrated, will probably permanently change your machine, such that it will be difficult to revert to hand-held mode.
You will have to remove the rotating height-adjustment turret, and even drill through the base below it, for the long bolt. I suppose you could tap a thread in the hole and re-install the turret, should you change your mind.
Also, you may have to remove Part 90 - I think the gear teeth may not permit passage of the threaded rod, and the threads on the rod may damage the nylon gear teeth. You may even want to insert some thin-walled aluminium tubing, to ensure that the threads on the rod do not engage with some internal ridges in the depth-adjustment housing.
It should be possible to fit a nut onto the rod where it comes out of the router top cover, as in my photos. Use a washer or two to reduce friction with the housing. I used a coupling nut, but it is not strictly necessary - it just gave me more freedom to weld on a lug to prevent rotation of the nut. If you cannot weld, you could drill through the wall of the nut at an angle, and insert a metal pin to stop the rotation.
Alternatively, you could reduce the end of the threaded rod, and thread it with the same thread as the bolt holding the turret in place. Then you could adjust the height quite conveniently from under the table, using a longish hollow handle with a nut on the end of the tube. I made such a conversion for a friend of mine, who has an AEG router that is fairly similar to yours in design.
I have a large Ryobi router that already came with such a feature, and I have not found it necessary to modify it, even if it does not have the élan and éclat of above table adjustment with a cordless drill.
The bit changing problem is less than you think: normally, you want the bit to be fully inserted in the collet, minus about 2mm. A number of the guys here advocate placing a 2-3mm O-ring inside the collet, and I have followed their advice. Then it is only necessary to drop the bit into the collet, and you have one hand to press in the button (under the table) and another to use the spanner (above the table). Some of the guys here hate that idea, but I am OK with it. The spindle-lock button should have a spring to disengage it when you release the pressure - I think part 14. In any case, I have developed the habit of rotating by hand, to make sure all is all right, before switching on.