If character 4 winds up being Pokémon, a worthwhile consideration for "why did they only wait until the back half of the pass to add it" is to think at it from a different angle.
SwSh moved 1.3M last quarter despite being a one year old game at this point. If you consider the fact that sales are also fueled by the holiday drive, it makes a lot of sense to have a Pokémon character be the one that's aimed right after the holidays. Many people are just now getting their Switch, and those newcomers who played SwSh will be able to see "hey look, a character from the game I got over the holidays is gonna get into this other game". Nintendo aren't in this business without full consideration of how they expect their software to sell, and Gen 8 in general constitutes one of the expected evergreens that still doesn't have playable representation in Smash.
There's also a lot of other factors to consider; "why wasn't a Pokémon out before Min Min?" It makes sense to get Min Min out first because by the time she's repped, her game would've already been out long after it's been in the mainstream, and it also kicks off Smash DLC with the novelty of a new franchise entering the fray. SwSh would've had a marketing cycle persistent throughout the year with its own DLC so it would have plenty of time to still have mindshare while Smash is running concurrently. After Min Min, they go for the big third party hitter Steve to get the mainstream press and maximize DLC sales further. Then you get to character 3; by that point you could entertain throwing a Pokémon in there, but you're also looking at the tentative schedule of the character releasing in Q4, which is the holiday period. You can also anticipate that maybe, you can secure a slot for another big marketing power move through The Game Awards. Go for Sephiroth, who this year received a remake of the most coveted JRPG in history. Now you've got three more slots left that would be expected to cover 2021, with the earliest one being shortly after the Holidays when Nintendo's no doubt pushed a whole lot of new games and consoles. Oh and lest we forget, Pokémon is going to be celebrating a major anniversary milestone this year, and holding off a character for Smash for this purpose is as good of an excuse as any when the marketing apparatus is gonna be firing on all cylinders this year.
Do I think a Pokémon is super likely? Not exactly, but I'd like to brace for the possibility that it's gonna happen because people really like to get their hopes up that it won't happen, and that's how they get themselves disappointed. I also think if a Pokémon does happen, the best bet is for it to be the next character because it's kind of the most appropriate period to go for. If a new Pokémon fails to be the next character, well, then I won't be so sure, but I'll still hold onto my expectations just to be safe and not get disappointed when the game fails to add whatever other dream candidate that got encroached.