Is it worth making my own tonic?
I think most projects like this are worth a shot at least once. You don't need a fancy kit at all. The process is easy enough and the only hard to find ingredient is cinchona bark which is easy and cheap enough to order online.
If you've tried the fancy tonic syrups then you have a good idea of if you'll like it and it will definitely be cheaper. I find most store bought tonic too sweet and one-dimensional and while I do like fever tree and occasionally buy, it's pricey. With homemade you can tailor the spices and bitterness level to your preference. I prefer half the citric acid of Jeffrey Morganthaler's recipe and a little less sweetener, making it crisp, bitter, and bracing but not over the top.
This has been discussed lots on the Spirits board. Here are just a few links. If you search, you can probably find some more.