My DD has been behind at school from the start of primary, she is winter born so not the youngest one. She is probably behind 1 year in her learning and maturity. I don't know how will she cope with secondary school in 1.5 years. I think keeping her behind one year will help her maturity and confidence and she will be more ready for secondary school. She has also lost lots of education, by spring this year it would be 2 terms, so nearly a whole school year.
I understand this is not normally possible in the UK, however I read an article that because of the pandemic your child could repeat one year if approved by the head teacher.
We have been home schooling her at home and can not see how she will cope with secondary school.