I am thinking I would get just a few plants to start the cycle. Then add plants as I learn more and develop a plan for the tank. Also probably add some rocks and driftwood. But I am also wondering about the impact on the substrate of adding plants. Is it better to plant everything at once? Or can I start with a few plants and add more over time.
You'll definitely want to add all the plants at once. Lots of plants during cycling helps combat the risk of algae outbreak which is more likely to happen during cycling by providing competition. Also, plants will uptake Ammonia as a nutrient, they prefer Ammonia before Nitrates as their Nitrogen source. Another positive thing plants provide during the cycling process is getting rid of Nitrates which will start to build up as well because they will uptake those too. With a cycled tank having enough nitrifying bacteria to break down Ammonia into Nitrates and then enough plants in my tank to uptake and remove those Nitrates I've gone 6 months without a water change in my 2.5 gallon tank. A heavy and diverse range of plants in a tank along with other diversity in the system like snails and shrimp to break down uneaten food, consume algae and break down fish waste and dead plant matter into nutrients can create a very balanced system.
I recommend cycling your tank with your fish in it. The only reason you wouldn't want to do this is that if you are introducing so many fish that the ammonia builds up to higher than 3ppm before a 24 hour period. Most of the time this isn't the case. I use something called "biological booster" it's a liquid that you add that has millions of pre-loaded nitrifying bacteria in it to give your tank a jump start with the process. You'll need to test ammonia everyday (First week I sometimes do AM PM testing for a few days so I know it can go 24 hrs without getting to dangerous levels) and perform a water change daily as well for the first week for sure, then after that test ammonia everyday and if it reads higher than .5ppm perform a water change. Third week, same thing, test ammonia everyday and only change the water if there is a reading on the test kit. You'll end up changing it 2-4 times the third week. Fourth week same as third week. As long as you are performing water changes like this (not letting ammonia reach dangerous levels) your fish will be fine.
I think you'll find managing one tank as a beginner enough fun and challenge. I would recommend focusing on one tank at first, getting that balanced and stable (which can take many months), you don't want to get overwhelmed and overlook one tank because of problems with another, it can happen easier than you think. For me, I was going to create a 5 gallon planted tank 3 months ago, but my 2.5 gallon tank started to have algae problems and show signs of nutrient deficiency. Troubleshooting and solving those two problems took all my attention away from planning my 5 gallon tank because I was so focused on fixing the issues in the other tank. I didn't have a choice either, you can't put algae and nutrient deficiency (or any problem really) on hold because if untreated will just get worse and harder to solve. After a month of testing various variables I finally found the problem to be phosphate deficiency. Plants did better and algae went away. A word of warning, problems in this hobby VERY RARELY come with an easy fix. Each aquarium is it's own complex ecosystem.
Substrate, filtration, lighting and first plants are the main decisions I have to make now.
Lighting is an important question, so is substrate. Depending on whether or not you go with high light or not will change how you manage your tank. High light will require some form of Carbon fertilization, whether that be with pressurized CO2 or liquid Carbon, which is, as many will point out, definitely not as good as pressurized CO2 and some plants don't like it -it will kill them. I use it myself in my small 2.5 gallon with high light and it works great. With high light and no form of Carbon input it's very easy to have algae problems, plus, if you're not going to use CO2 or liquid Carbon why even have high light? It's not like the plants can utilize it for photosynthesis because they require CO2.
I would recommend an active substrate for a beginner. An inert substrate requires fertilization because it contains nothing for the plants. Balancing a fertilization regime can be tricky sometimes but they do make good all-in-one fertilizers like Thrive. However, active substrates that are like soil will have many more benefits than inert substrate. You will not have to dose fertilizer, which means less maintenance (no daily or weekly dosing, less monitoring, measuring, and experimenting) and less expense (many hobbyists make their own in bulk due to the cost of fertilizing). Soil-like substrates are made to contain everything plants need but sometimes you do dose a little of this or that depending on plant needs and uptake speeds. Active soil-like substrates will also contain lots of beneficial bacteria that will do a host of great things for your tank as they break down organic matter within the rich substrate. The bacteria, while doing this, will also create lots of CO2 for your plants, much more than you might think. Think of the benefit of natural substrate as your own CO2 system. As Diana Walstad (creator of the Walstad Method and author of the book Ecology of the Planted Aquarium) discovered you can have a very heavily planted aquarium with no liquid carbon or pressurized CO2 injection. She found that most of the CO2 that was being created for the plants was being produced by bacteria in the substrate. She claims (I'm unsure if it has been proven) that nothing is better or provides more of what plants need than organic potting soil, and it makes sense, as it's real natural soil, and that's what plants live in in nature. However, I would not recommend using organic potting soil for your first tank, it can get messy. There are substrates that are "in the middle" of straight up potting soil and inert substrates out there, I use Fluval Stratum. Fluval Stratum is essentially soil but it's in little balls and compared to potting soil not messy or hard to work with in a tank. A downside to these soil-like substrates is that they will eventually run out of nutrients and need to be replaced, usually in about 2 years whereas an inert substrate, with proper cleaning and maintenance doesn't need to be changed because well, it's just rocks, gravel, sand or something similar.