Here's how I did my fishless cycle:
1. Setup all equipment
2. Fill with treated water
3. Dose pure ammonia (in my case, I used Fritz Pro Ammonium Chloride powder) until I got to 4ppm of ammonia
4. Test the water 2x daily (for my schedule it was every morning and night). For the first week or so, I only used the ammonia test since I didn't see any change to the values.
5. Once I started to see ammonia dropping, I would dose the ammonia every evening (more convenient for me) until I got it back up to 4ppm. At this point, I also started testing for nitrite.
6. Continue this process every day until the 4ppm of ammonia dosed in the evening was gone at the 24 hour mark. At that point, I know my first level of the cycle is 100% complete. At this point, I kept dosing the 4ppm until I saw the nitrites also hit 0ppm.
7. Once nitrites hit 0ppm, I continued dosing until I saw 0ppm ammonia and nitrite after dosing 4ppm 24 hours earlier. Once I saw that, I knew my cycle was complete.
8. I dialed back my dosing to 2ppm ammonia every 24 hours until my fish were scheduled to arrive. The night before they were to be delivered, I changed nearly 100% of the water with fresh aged and treated water. This removed all of the nitrates that had bulit up during the entire cycling process. I tested the water for nitrates just before the large WC to see what it was, and it was definitely above the max on the API kit. After the water change, it was <5ppm. Fish arrived the next morning around 9:30AM, so they had a super clean and well cycled tank. I did not experience the "mini-cycle" that some experience when adding fish.
Some things I would consider changing about your current regimen. I would stop doing water changes. During the fishless cycle, it's not necessary, and you're only diluting the amount of ammonia/nitrite available. I would do what I've done - hold off on a water change until the very end right before your fish arrive. This will keep your parameters even throughout the cycle and you can easily eliminate any level of nitrates by just removing all the water. Plants don't necessarily help your cycle. At a very small level, live plants can absorb some levels of ammonia and nitrite, but they mostly use nitrate for their energy. While cycling with plants is possible, I don't do it because I like my cycle to be as simple as possible (don't have to dose ferts, I can keep the lights off, etc). I don't think the plants will harm your cycle.
You really need a test kit and test your water at least once a day. How else will you know if you need to add more ammonia or not? You should only add ammonia to get back to your target PPM (whether that is 2, 3, or 4ppm).
If you can, I would put off the shipment of fish for as long as you can until you know for certain your tank is cycled. Juvenile discus are not forgiving and you will more than likely have issues with ammonia/nitrite due to the frequency and amount of food you need to feed juveniles for growing them out. Likewise, if your 55 is going to just be a grow-out tank, I would consider removing any decor and substrate, leaving them a bare aquarium. It'll make your life a lot easier to keep it clean.