Toolbox Theory is a special case - unlike the other updates, it was only really developed by 8 people handpicked by Pacifica himself. These coders (ranging from "Senior Contributor" to "Developer") created the mechanics of the update, which is pretty much the only part of it that requires any sort of actual knowledge, as the rest is just basic paint-by-numbers stuff any teenager can do.
As these coders are rather experienced and valuable team members, they can obviously get away with more shit than common low-level team members. In general, all coders have no quotas (since it's near impossible to quantify that), so all progress reports are subjectively judged by the team leads / lead devs, who obviously aren't impartial in the slightest. Combine this with the fact that no coders was exclusively delegated to TT development (they could still work on bug fixes and their own local dev teams), and you might begin to understand the delays that have plagued the update for months now.
From what I know, the update was supposed to be "code complete" by January 2021 - at this time, it was supposedly announced that all mechanics were finished and only had to be implanted by the low-level coders of the rest of the team, stuff like manually rewriting near every focus to function with the new system. This was also the first time most team members got to test the patch themselves, which probably explains how the "wait this is actually not fun at all" realization took so long to hit.