All kinds of things are unobservable save by their effects. There is lots of (indirect) evidence for black holes, including lots gathered after they were predicted. You're just plain wrong. What you say isn't true.

Also, you style yourself as a "structural philosopher", and yet you don't mention any structural philosophers, you don't seem familiar with the different flavors of structuralism, your descriptions are completely lacking standard philosophical terminology (e.g. epistemology, phenomonology) that would help clarify the exact point you're making, and you also seem largely unaware of what philosophers have written on the issue.

You bring up mathematics but make simple mistakes, you bring up physics without displaying any understanding of what the physics actually means beyond the most superficial cartoon version, and you charge people with ignoring approaches that in fact they use all the time in a far more sophisticated form to the extent that people rarely even talk about it any more.

The reason you can't convince people is that what you think you understand makes sense in your own head, presumably, but if it is founded on anything solid, you don't seem to be able to communicate it.

It's not that the concept is too hard to understand. It's that it's too simple to be right, and therefore isn't convincing. "Let's keep it simple" indeed! No, let's not keep it that simple. In the words of Einstein, make everything as simple as possible but no simpler. When challenged with "what about this thing that doesn't fit with the suuuuper simple picture?" you don't show why it works; instead you show some other overly simplistic poorly-fitting analogy. In order to know whether "spacetime" or "spacetime+" is better, you need to know something about the +, something about the "condition for matter".

What we do know and/or think we know is amazing and glorious and wondrous, and if it turns out that you do presently have any insights that others have missed, you would do well to learn a whole bunch of it and be able to talk about it competently--and anyway, it can be deeply fascinating and can help you develop more insights that are novel and can also be communicated, so there isn't much to lose.