lol if you have an infestation attacking your roots, then yes, it is most likely that.
very unlikely nutrition related, as the plant is relatively healthy otherwise.
if it is something that lays larva in substrate, Bti should be effective. thrips don't reproduce that way, from what i know? but that's reference info you can look up. Can use pyrethrines to kill thrips. it's contact only. Don't spray on flower. Bti is safe anytime