Again I think the whole late DP era may have seemed filler due to what a slog of formula it was, even in the episodes that had some vague continuity. Slapping a small bit of progression onto the end of an otherwise monotonous episode doesn't really automatically improve its entertainment value.
Examining the 38 non-fillers between Snowpoint and Sunyshore to determine whether they were truly non-fillers, or whether they were semi-fillers.
Semi-filler = Formulaic episode that has a small bit of progression slapped onto the end of an otherwise monotonous episode
1. A Pyramiding Rage! True non-filler: Paul vs. Brandon
2. Pillars of Friendship! True non-filler: Pokémon Hunter J / Brandon / Regigigas episode
3. Frozen on Their Tracks! Semi-filler
Looker does debut in this episode, but to an anime-only viewer, with no game context, he seems like a COTD and the overall plot of the episode is fillerish. Until Team Galactic's logo appears in the final seconds of the episode, it's not clear this episode is relevant at all.
4-6. Ash vs Paul Lake Acuity, plus the episode where Dawn cheers him up. All 3 true non-filler.
Pedal to the Mettle!
Evolving Strategies!
Uncrushing Defeat!
7. Promoting Healthy Tangrowth! Semi-filler. Dawn's Mamoswine learning Hidden Power is literally the definition of "slapping a small bit of progression"
8. Gateway to Ruin! True non-filler. Mt. Coronet / Team Galactic / Looker episode
9. Three Sides to Every Story! Semi-filler. Johanna inviting them to the Twinleaf Festival is literally the definition of "slapping a small bit of progression"
10. Strategy Begins at Home! True non-filler. Important episode in Dawn's arc & in terms of Dawn's relationship with her mother
11-12. DP140-1. Semi-fillers, because they had fillerish plots and Palmer ends up being a COTD at the end of the day since it never set up a Sinnoh Battle Frontier
Historical Mystery Tour!
Challenging a Towering Figure!
13. An Egg Scramble! True non-filler. Johto Festa begins, they meet Lyra and Khoury, Dawn gets egg and hatches Cyndaquil
14. Dressed for Jess Success!! True non-filler. Whole episode is taken up by a contest, which James wins for Jessie
15. Bagged Then Tagged! Semi-filler. The Johto Festa ending is literally the definition of "slapping a small bit of progression"
16. Unlocking the Red Chain of Events! True non-filler
17. The Needs of the Three! True non-filler
18. The Battle Finale of Legend! True non-filler
Team Galactic finale 3-parter
19. Mastering Current Events! True non-filler. Air Master training episode for Gliscor
20. Double-Time Battle Training! True non-filler. Whole episode is contest related. Zoey wins fifth Ribbon. Dawn training episode including development of Flame Ice combination
21-22. DP156-7. True non-fillers. 2-parter in which Ash catches Gible
A Meteoric Rise to Excellence!
Gotta Get a Gible!
23. Short and to the Punch! True non-filler. Buizel learns Ice Punch, and the whole episode is dedicated to training for it
24. Yes in Dee-Dee It's Dawn! True non-filler.
25. Playing the Performance Encore! True non-filler.
Dawn 5th ribbon 2-parter
26. Fighting Ire with Fire! True non-filler. Paul and Barry both appear, also Monferno evolves into Infernape.
27. Piplup, Up, and Away! This could be seen either way. While Piplup learning Hydro Pump is kinda "tacked on" to the end of the episode, I do feel the plot of the episode isn't really fillerish. It is basically the finale of the long-building confrontation between Piplup and Gible. However, to give you the benefit of the doubt, I will count this one as a semi-filler.
28. Flint Sparks the Fire! True non-filler. Introduces Flint and Volkner, Ash battles Flint, and Volkner comes out of his depression
29. The Fleeing Tower of Sunyshore! Semi-filler. Very fillerish TR plot, and the only meaningful thing that happens is that Volkner says come back after Grand Festival
30. Teaching the Student Teacher! Semi-filler. Jessie wins her fourth ribbon and Dawn perfects Ice Chandelier, but both of these are tacked onto a fillerish plot.
31. Keeping in Top Forme! Semi-filler. It's not a filler only because it debuts stat trainer Marley
32. An Elite Coverup! True non-filler. Ash battles Bertha
33. Dawn of a Royal Day! True non-filler. Whole episode is spent on a contest, which Jessie wins. Dawn also gets Togekiss in this episode
34. With the Easiest of Grace! True non-filler. Whole episode is a training episode for Dawn to get Togekiss into contest-ready shape.
35. Last Call!! First Round!! True non-filler.
36. Opposites Interact! True non-filler.
37. Coming Full Festival Circle! True non-filler.
38. A Grand Fight for Winning! True non-filler.
Grand Festival 4-parter
So based on this analysis, out of the 38 non-fillers during the "year-long gap", 28 are true non-fillers, and 10 are semi-fillers.
In total, out of the 51 episodes in the "year-long gap",
13 fillers
10 semi-fillers
28 fully meaningful episodes