I converted two of our old diamondbacks to electric, but they already had front shocks. These were the spring loaded "pogo sticks" common on bikes in my price range. What do I care, they work fine for rolling off a curb or the occasional small pothole.

I sympathize with your situation. My first conversion was a old Trek, with a 1" threaded fork, I also wanted a suspension fork (to be like the diamondbacks which were not converted til a year later). I finally found a 1" threaded fork, the RST-Capa like this one https://carsoncitybikeshop.com/fork-...-50-bk-disc-v/

What I discovered was that this fork did not have the threads cut deep enough to fit my Trek. I tried to cut them with a pipe die, but it's hard steel and you need a bike tool to do the job, In the end, I converted the Trek to a threadless mount, which required some extra parts. I would suggest getting the RST 1" threadless fork, if you can find one, and convert the Diamondback to threadless. In my case, I used a new headset and replaced the cups in the frame. A headset is 20 bucks, and the rest of the parts are about the same.

The advantage of a new fork is that it will have disk brake mounts. A rotor/caliper is around $50. Your ebike kit will have a new brake lever, but you'll need a new front cable/sheath. Avid BB7 are what I have used, after wasting money on no-name calipers, and the lesser Avid BB5. However, today, I would try the Zoom HB100 calipers. They are hydraulic pistons, which means they self adjust, but use a cable to actuate. I've got two in use this year, but I hear they leak.

The disadvantage of a suspension fork is that it raises the front of the bike 4-5 inches. If the frame was not designed for this, it will affect the ride/handling but I got used to it after a day. I'm not used to the middle of the bike going up 2 inches, My seat on the Trek is all the way down, and my toes just reach the ground. I also had to get a longer kickstand.

As for your derailleur, I have swapped them, but only on bikes where the derailleur screws into the hanger. Many of the low end Shimanos have the derailleur riveted to the hanger, so it's difficult to change unless I find one withj the same hanger.