Tetra SafeStart worked for me on multiple occasions. The cycle never crashed afterward. If something caused ammonia/nitrite spike in your case, it must be either a mini-cycle due to too much ammonia was produced, or something else was causing the crash. Tetra Safestart is proven to have the right type of bacteria to keep the filter media cycled without the need to keep dosing the product. Some other products have the wrong type of bacteria which will drown in the water after a week or two, and causing the cycle to crash if you don't keep using their product.
I'd think mix up the RO and tap water is too much work. More calculation... lol I am lazy... but it is indeed easy to make mistakes when more calculations are involved.
A 10~20% partial water change is considered a small water change. A 30~50% partial water change is the typical recommended amount. You can do 20% partial water change twice a week to make up for it if you wish to keep the environment change as small as possible for your fish. Then again, I'd think any water change more than once per week is too much work, but that's just me.
If memory serves right, a general rule was that you need 2w of lights per gallon of water for high demanding plants. Whatever rules still have limitations. Yes, 20" deep is pretty deep lol. The height of a tank does matter.
On a side note, I'd like to recommend not to go for the "high tech" route for plants. That is, high intensity lights plus CO2. It is really troublesome. Sure, it is awesome to look at some planted aquariums. The amount of "gardening" work behind it is enormous.
