This explanation also explains why both Smash 4's DLC and the Fighter's Pass were releasing content literally up until days of their respective deadlines. When they say everything will release by 2022, I feel that's the same as them saying that they're going to release stuff up until that point. I don't think they're going to release FP 11 in September, clasp their hands together and be like "welp, we're done", I think they have things planned out up until New Year's 2022.
If they want to work until the end, I would recommend to temper a bit the expectations about how many “bonus” fighters we would get. Remember that a new fighter takes between 6 months to 1 year of development (i suppose this includes planning, design, programming and lots of testing), a mean of 9 months per fighter is very pausible. It all depends on the complexity of the fighter and how many stuff they can recycle.
With FP1, they worked 3 characters at the same time. With FP2, its likely they are doing the same. So, nowdays, they are full on board with CP9, working on CP10 and starting CP11.
So well, let’s say that CP9 will release around march (8-9 months after Min Min and next Amiibo wave is end of that month), CP10 by June (late month, after E3) and CP11 by September. The CP6/9 team would have been “free” around 9 months earlier than the end date, that is enough time to develop a new fighter (don’t expect a full CP, just the fighter with one new remix, no stage or spirit board) for a december release. Meanwhile, CP7/10 team would have 5/6 months left, that is not enough time to develop a completely new fighter, unless its a semi-echo, like for example Wolf. CP8/11 can help the other teams to speed up things a bit after they finished.
In summary, if its possible to get some bonus, expect 1/2 fighters (no stage or spirit board, maybe 1 new remix). One can be a full original fighter (without complex mechanics like Steve or Min Min), the other must be a semi-echo yes or yes (i think a good candidate for that would be dixie kong as she can borrowed many things from Diddy kong to make her development more quickly).
As you have been saying, december is an important date for smash (and its true), so maybe by next december, we can get a final smash presentation showcasing this two last fighters and a bonus mii costume wave.
PS: Talking about Chun-Li, IIRC, there is a user here called xenother that commented on gamefaqs a long-time ago (2018, before Ultimate was on the streets) that Chun-Li was considered for base game but sadly scrapped.