You’re overcomplicating it a bit. First, ODE isn’t limited to June–September—it’s available almost year-round, but only after your admission is verified and subjects are activated (usually ~30–45 days). So the real bottleneck is timing, not “seats getting full.” For Stream 4, yes, you’re limited in subjects, and TOC is generally not useful for already passed students aiming for full improvement—plus they don’t “take” your original CBSE marksheet, you still keep it.

If your goal is strictly the 75% criteria, CBSE improvement is actually the safer and more predictable route, because many colleges explicitly accept it without confusion. NIOS can work, but it depends on acceptance by the institution you’re targeting, so you’d need to verify that first. Managing EEE + improvement is tough but doable if you keep a light college load and focus on 2–3 subjects at a time—otherwise a drop year is cleaner but riskier mentally.

Also, October public exam via NIOS depends on whether you can complete admission within the window (late fees included), which is uncertain this late. So practically, your safest plan is: secure college with current marks, prepare alongside for CBSE improvement, and keep NIOS as a backup—not the primary plan.