Edit: after reading your post again, I get the feeling that my advice may not be relevant to you. My comment is specifically dealing with my experience as an American.

If I go to a community college, I could improve my grades, but the material they cover is a replacement for high school classes and I’d be precluded from signing up for entry classes at the four year college.

This system is pretty incongruous with my college experience. You may get more accurate help if you specify what area you’re from

Are you a current high-school student? I can only speak from my own experience about a decade ago but I’d recommend to take as many AP and dual-enrollment courses as you can. AP courses were weighted and boosted my GPA way up and dual-enrollment let me take college courses for free in my highschool. If your school offers either of these, take as many as you can. My local state university actually partnered with our community college so transferring was as smooth as could be. It might be worth actually chatting with both the community college you’re considering and your eventual four-year university’s admission offices and making sure that what you understand is right.

Don’t discount the benefits of Community College, either.

When I tried CC, the material was absolutely high school level just with smaller font in the textbooks.

This is absolutely the case with freshman courses at a university. Only difference is that you’re paying three times as much for tuition.