Short answer: Take a look at the Witchwar Legacy. If you don't like it, you're out of luck.
So, one if the bigger issues you're going to be facing after completing the Witch Queen's Revenge is that your party is going to be really close to 17th level, if not going on 18th. At 17th level you are going to be running some very high level play, so there aren't going to be a lot of modules designed with that power level already built in. Luckily, the Witchwar Legacy is of the right level, and even set in/during a retelling of the same events, but does not fill in the gaps in pgs 66-68. If you like the Continuing the Campaign section's details regarding Baba Yaga, you probably aren't going to want this module (except maybe to reskin it as one of the Collector's locations), but no existing modules deal with any of the Winter Collector locations explicitly, and the best approach in that case is going to be to design the adventures yourself. If the Collectors aren't a requirement, I'd run the Witchwar Legacy, mod the story slightly to account for recent events (maybe have Baba Yaga's ending of Evanna's ritual combined with everything else so weaken the enchantment on Tashanna that the daughter has managed to break free of her enchantments and return to life in her necropolis. Use Tashanna herself in place of the Torc as the MacGuffin that must be recovered (continuing the lesser of two evils trend, only Tashanna, who's had the last 1000 years to think, knows how to kill her mother) and award levels and/or Mythic tier along that module.
If you would rather to revamp low level campaigns for what is effectively epic-level play, I'd recommend the Shattered Star adventure path, which has an excellent 5-adventure build up to a huge climatic battle, but is by no means designed for epic play. You would have to re-do the monsters and traps in every dungeon and also re-skin the fluff to pertain to the Queen of Witches'. This would involve a lot of work. The advantage to be gained from this is that the dungeon level design and basic encounter themes can be largely preserved, and the low-level NPCs reskinned as the various low-level entities mentioned in the continuing the campaign section (you should note that other than the great wyrm white dragon hive planet, none of these locations should provide any challenge on their own against the PCs and, in fact, most of these inhabitants would probably be happy to see the thing go (it eats their cold, which all of the inhabited locations want more of), except that the Queen of Witches might want them dead as a result, if they know about the collectors' true purpose). It also has 6 modules, with six dungeons, which nicely corresponds to the needed number of winter collectors. Nonetheless, even using these modules you are likely to be basically making an 'inspired by' adventure series rather than really adapting a module.
In any case, the Dear Grandmother's stats are given here and on pg. 74 of your final module. They look pretty solid, even for epic play, and should trounce your upstart PCs if they aren't careful (which is appropriate for 20th+ level players who enjoyed Reign of Winter, I think). Your players should understand that they are basically taking on a God in opposing Baba Yaga and realize that they will have to take out all the stops to pull this one off. In actuality this feat is certainly feasible at 17th level, but giving the PCs another module or two to prepare for the final encounter can help stress both that Baba Yaga basically got away scotch free and let them finish levelling up enough to be a threat if they are underpowered.