Thargoids imho are a season by themselves (lore, worlds/region, technology/ships, IA) so i don't think they will appear soon.
FD will probaby introduce the human scale as soon as our avatars will come (Ground crew on starports and settlements, first steps of walk around before 3.0)
This isn't a matter of opinion but of speculation. FDev might not share your opinion on what qualifies for a season.
Not to mention that Thargoids have been in development since before release, the lore most certainly has been finished.
It was my understanding that parallel teams were working on each release. Meaning the others are not necessarily delayed unless they had to re-task devs to put out the 2.1 fire.
Clearly, we won't know until FDev feels confident enough about a release date.
I am thinking more or less along the same lines as you. I'm certain the whole team wasn't working on 2.1, it's fairly light on artwork(engineer bases were probably finished prior to the delay), not to mention that the faction avatar portion of 1.6 would carry over into 2.3s character creator.
In fact, a lot of the updates were pretty well along when they showed them off last year.
Meaning the majority of the work in 2.1(during the delay) was probably UI and under-the-hood stuff.
Still, though, until we see word from FDev we can only speculate.