I'm not really following your logic. Apple could in fact make a larger iMac, but a desktop version of the MacBook Pro would be an iMac Pro.
I don't think that Apple would EVER release a Mac mini Pro. It completely defeats the purpose of the Studio. The Studio is the pro version of the Mac mini.
For the rest of what you wrote I can't really decipher it. There is only a jump between the iMac 24" and Mac mini and Studio Mac because you are creating one. Apple could easily adapt the philosophy of - 24" iMac and Mac mini for consumers and more budget conscious people. Studio Mac with external display for those that want more power and a larger screen of their choice. Mac Pro for professionals. This idea that they HAVE to have some model in between doesn't make much sense to me. Its not saying they won't but I could easily see them keeping things the way they are now for a while.
Yes, there is a leap in desktop.
The "semi-professional" processor, the M1 Pro, is missing.
The M1 Pro I think for many people is fast and powerful enough for "demanding" tasks, without having to go to a much more powerful (and expensive) M1 Max. The M1, especially in GPU, falls short for certain people (myself included), and I think that processor on desktop would be valued by many people, either in a Mac mini (replacing the space gray Mac mini), or in a "Pro" iMac.
The 2017 iMac Pro had Intel Xeon processors, which are what the Mac Pro can carry. It was a transitional replacement between the "trash-can" Mac Pro and the current Mac Pro.
The "Pro" iMac referred to today, is not an all in one with an M1 Ultra, for example, which would be something similar to what we had in 2017. But to a larger and more powerful version, of the 24" iMac M1. With Apple Silicon, this iMac "Pro" would cover the desktop space of a MacBook Pro, with identical processors, but below the Mac Studio.
On the desktop, there is currently no "intermediate" computer between consumer and professional computers, because there is no such gap with the "M1 Pro", which is currently only available in one model (MacBook Pro). On the desktop, either with the "high-end" Mac mini (it won't be called Mac mini Pro, obviously, but it would replace the space gray Mac mini that came out some time ago to fill that "semi-professional" gap), or with the iMac "Pro" (it could be called Pro, but it doesn't necessarily have to be called that way either, being simply iMac 27" or 30", with more sober colors and it would already be, but in this one, from a marketing point of view, it would make sense to use the "Pro" name, since it would use the same processors as the MacBook "Pro").
But Apple has always had a "semi-pro" niche within its product line.
Entry (or consumer) models. MacBook/MacBook Air/MacBook Pro 13", Mac mini, 21.5"/24" iMac.
Semi-professional models: MacBook Pro 14"/16", Mac mini high-end and iMac 27".
Professional models. MacBook Pro 14"/16" high-end, Mac Studio, Mac Pro.
As of today, in this "semi-professional" gap there are two models missing, which can be covered by one model, either the Mac mini or the iMac, or both, but I think that this gap remains to be filled on the desktop, because today there are only "consumer" or "professional" models.