The number looks significant because you've added all the numbers together, but the actual effect of these updates is much lower. Endurance won't help against slashing weapons, and the usefulness of Dexterity has never even been proven, so that leaves only Strength, Agility and partially Magic.
LN saya "sometimes winning, sometimes losing" so I don't know what it is if it's not 50/50. Even if Bell was a more frequent winner, it's really just a headcanon since it's not stated or implied. I agree that Bell with final level 4 status would have won 9 or 10 times out of 10, however that doesn't negate Van's feat.
What does Hogni have to do with this? The original Tsubaki is only level 5, and even if she's close to leveling up, her combat skills are at best the same as the new level 6. Hogni is stated to be competing with Ottar for 1st or 2nd place in terms of melee combat. Ottar's combat skills are close to the new level 8. This makes the difference in combat skills 2 levels. Using a strong level 6 with such mastery to compare to a pseudo level 6 is a bad example.
Kaguya would be stronger, but that doesn't negate Tsubaki's merit in any way. Kaguya is known for being able to defeat a level 4 being a level 3 with the support of a level 2 prum. Her techniques directly take her to the next level of threat. Being weaker than her is no disgrace.
in fact, the reason for that statement was something else entirely, not personal power: "The spirit warriors that had been so effective against Freya Familia were Apate Familia’s secret weapon. However, they needed adjusting after every battle by a team of highly skilled mages and hexers. Otherwise, the adventurer’s physical body and the spirit it was infused with would begin to reject each other, leading to mental disturbances, physical breakdown of the flesh, and eventually, complete loss of control. At that point, even Basram’s ringed staff would no longer be able to exert influence over them. Basram’s words, that there was no substitute for a purebred first-class adventurer, rang true. No amount of trickery could reproduce their power without running into issues." While I agree that they are generally inferior to full-fledged level 5 adventurers, they should still have no problem beating a normal level 4 based on what we know about the level difference. They are stated to retain some of their techniques, and have animal instincts, regeneration, and instant magic, and on top of that a level 5 status. much more than what the current Raul would offer. yet Tsubaki won, even without the OP skills and magic. she is far, far above average just with her fighting prowess.