Hello!
I don’t necessarily think that it is too late - even if invites to later rounds have been sent out, it is likely that they will review all applications that have been submitted before the deadline (and there is plenty of chance to still be progressed).
The most important factor is whether you can submit a strong and tailored application - therefore, if you believe you can do that for the spring deadline, I would encourage you to apply if that scheme is more suitable.
However, if you feel you may benefit from some additional time to refine your application (and aren’t too fussed on the particular scheme), I would aim to submit early for the summer scheme.
Best of luck!