I think your mix was too concentrated for such a young plant.
Sprays to kill insects are often harsh and if you use the recommended dose you often run into burning issues, especially on younger leaves.
I tried half doses with what I got a few years ago and then it was soft enough to not burn the plant, but it didn't kill the bugs either, so I avoid spraying for pests.
It's definitely not from topping, the burn is on the top and lower leaves and random. She is in early stage of N toxicity, so there is no need for her to cannibalize her old growth and the plant is young, so I would rule that out and lower your feeding.
Using predatory mites is the least impact for your plant and extremely effective. Just make sure you identify the pest correctly because a lot of them are very specialised and one to control spidermites often won't eat thrips.