I continue to think that adding more states is more likely to occur during the next Democratic administration compared to adding more Supreme Court Justices. But it will be controversial, and, following Machiavelli, anything especially controversial should be done at the very outset of one's tenure as a ruler.* What that means is we want any new state admissions to be part of H.R. 1 (which most people already expect to be a voting rights bill). And in particular, we want the new states set to be added to be ready to go on inauguration day.
This is especially important if we want to extend statehood beyond the most obvious candidate, D.C.. Puerto Rico is a complicated case because statehood has been actively debated there and remains controversial. But there seems to be relatively little discussion of statehood for other American territories, such as Guam or the U.S. Virgin Islands. Yet if those territories also were ready to announce, on day one of a Biden administration, that they were applying for statehood, it would be much easier to roll them into a larger bill than trying to mobilize them on the fly.
* Machiavelli also suggests delegating the task to an underling and then, once it's complete, executing him in a high-profile fashion. Not all of his advice is applicable to the modern day.