If the sidequests are referring to them and also that all 25 are the Legends (I dunno about that, necessarily), it's possible some of those quests might have different options (either interchangable like the Galar horses, or forced like VE) or contain multiples
For the latter, imagine if we had a repeat of some of Crown Tundra and one sidequest was for like the 5 Regi and another was for the birds and so on. Meanwhile a quest for the Lati could be shared into one quest internally but have a different reward, or like emerald you have to choose one.
I'm sure Khu will throw out more hints that'll clarify it moving forward and playing with numbers is always easy to make things fit, but maybe something like...
- Birds
- Beasts
- Regi x5 (+Regigigas?)
- Lati
- Lake spirits
- Musketeers
- Tapu
- Forces of Nature could be all one big sidequest, like Legends. They don't necessarily have to make Tornadus & Thundurus VE again, if that matters any. BDSP let you get both Lati, and both Kyogre/Groudon isntead of putting them as VEs (instead opting for a beast/ho-oh and bird/lugia split)
- Mewtwo
- Lugia/Ho-oh
- Kyogre/Groudon
- Rayquaza
- Dialga/Palkia
- Giratina
- Reshiram/Zekrom
- Kyurem
- Xerneas/Yveltal
- Zygarde
- Cosmog (and maybe this time it can be shiny!)
- Necrozma
- Zacian/Zamazenta
- Eternatus
- Heatran
- Cresselia
- Kubfu
And then Type Null gets left out since the UBs are? Or something like that. There's various options. Kubfu & Type Null could be a gift, they could exclude someone else, beasts & birds could be the "same" sidequest internally but different based on version, they're more aggressive about grouping things together and there's quests for other things, etc.