We also don't have any proof that any other member of the Big Mom pirates including the older siblings is of comparable strength to the sweet commanders

Lack of a negative does not prove a positive.

they would be included in that ranking or would be known as commanders rather than just ministers

Unless the Ministers have a set territory they reign in Big Mom's name and the Sweet Commanders are her field commanders (that she sends all over the place), thus creating two categories that need a name.

Again, there is no evidence that the Commanders are stronger than all of their siblings.

There is and should be a clear difference in power

That's only your personal opinion, not evidence.

And about Blackbeard he wasn't a commander and never took the tittle because he wanted to conceal himself and lay low untill the perfect timing to execute his plan this was also explained by Shanks

And a Big Mom sibling could not want to just not be bothered by all the annoying tasks that comes with Sweet Commander title? Or have any other number of reasons for wanting to stay out of the radar?

Unless there is a serious rebellion against Big Mom

... You do realize that a LARGE theme of this arc has been that a ton of the Big Mom kids hate their mom, only follow her out of fear, and would not hesitate to jump ship, right?

Ask yourself this then how would Daifuku or Oven would fair against a fresh Gear fourth Luffy?

We do not know enough about either of them to say.

But neither do you know enough to say that it is not the case that Daifuku could be on a very similar level to Katakuri, Smoothie, and Cracker.

There's no evidence against it.