It sounds like you are needing a “traditional project”. Back when I joined iNat 5 years ago projects had to have observations added to them manually, and there was a way you could set them up to automatically receive observations. I think it became apparent that as iNat grows, that way of handling projects would become increasingly server intensive and problematic, so they have been phased out to a degree by the new form of projects (collection and umbrella). The way these new projects work is to effectively create a search filter with a fancy front page to it.
The advantage of these new project types is that they are “auto add”, once set up you don’t need to add observations to them, but that brings it’s own set of downsides… for one, you can’t selectively exclude observations that meet the criteria.
When you go to create a new project you are presented with the two new choices, and way down the bottom in small text is a link to the old “traditional project” type…
I think it will suit your use case much better than the auto add collection types.
Here is an example of my own that I created when I joined iNat:
note that it’s been over a year since I added to that project (the downside is you have to MANUALLY add!) and my interest in it was tapering off long before then.
Not long after creating it, it became apparent that many other iNatters have created similar, and I briefly considered trying to organise them all into a mega “back yard” project but prior to collection and umbrella projects that would have been a massive effort to get everyone working together. There have been similar attempts to that though, with “in my house” and similar…
anyway, a traditional project should meet your needs!