I think you have covered most of it. Your plants will help with ammonia but are not sufficient to carry most of the load. the addition of the new plants will help. just be aware Amazon swords get huge and if the tank is hospitable to plants, it will outgrow the tank in a matter of months.
Unless you are doing a high light co2 added tanks I would hold off on adding ferts. most plants come to us in great shape with stored nutrients. In addition, the ammonia and nitrate the tank produces are food for the plants. One of my favorite sites to send people just starting with plants the Tropica site. they are the world's leading developers and producers of top quality aquatic plants. The have excellent resources on their site that will guide you from A to Z with words pictures and videos. Here is the addy
http/tropica.com/en/guide/make-your-aquarium-a-success/ Take some time and click on all the links on the left side of the page in order and you wont regret it.
As for you ammonia levels and the fish. 4 ppm in pH 7.6 at 78F is very toxic to fish. Fortunately zebra danios can handle more ammonia than the average fish so it was a proper choice for a fish in cycle. You probably should have started with no more than 4 but that is water under the bridge. Your fish yold you how bad 4 ppm was in your parameters by their behavior.
A fish in cycle is like a dance. One the one hand you want to allow the ammonia to rise and fall on its own while at the same time you do not wish it to get high enough nor persist long enough to harm the fish. Any ammonia test that shows total ammonia does not provide enough info to know if that reading is a danger in a specific tank. For example if your tank had a pH of 7.0 that 4 ppm of ammonia should not be toxic and the fish should be able to handle it for a day or two. That said once one hits 2 ppm of total ammonia even when the NH3 involved is not at toxic levels it is important to monitor the fish. the arbitor of the possible ill effects is not merely our test results, which may or may not be accurate, but how the fish behave. Even if the test indicates the ammonia level is safe for the short term, if your fish show signs of ammonia poisoning, change the water.
The above, along with further issues, is why it is always preferred to do a fishless cycle rather than a fishin unless one has a very well planted tank. In these one can add fish gradually from the start and have things go fine. What you cannot do in such tanks is put in a full fish load all at once the way we can after completing a proper fishless cycle.
In your tank at a PH and temp above you need to keep total ammonia at 2 ppm or less. At 2.5 ppm you have crossed into the danger zone for many fish even for fairly short term exposure. Moreover, if you do get to the point where a water change is required, it is usually fine to do 50% and allow ammonia to persist at 1/2 those levels. This keeps it safer for the fish while still pushing the cycling process to its conclusion in the shortest reasonable time.
Next is a heads up for any nitrite you may see. While many folks urge water changes for nitrite, there is a better way. One can uses smaller amounts of salt to block the nitrite from being able to affect the fish. It is actually the chloride part of the salt which does this. Using some salt for a short time should not harm almost anything in a tank, even things that don't do well with it at higher amounts or for longer time periods. This lets one avoid the stressful potential of repeated large water changes in a tank where fish are not really settled in. If you get to this phase directions for using salt can be found in this article
http/www.fishforums.net/index.php?/topic/433778-rescuing-a-fish-in-cycle-gone-wild-part-il/
And again, all this is a great reason for doing fishless vs fish in whenever possible.