Hi.
You are correct in that a series mark has to have very minor differences, for example zoobks and ZOOBKS- this would be a series mark. I am not offering 100% certainty here but in my view- it will be viewed as a different mark and will not therefore be accepted as a series mark.
If you did apply for 2 and it is rejected as a series the UKIPO will give you the opportunity to delete one of them-you will not lose the whole application.
Hope that helps!