I would not worry about combining and then trying to “catch up”. Your kidos are still quite young, there is no rush 🙂
I would either continue using the BF books as you are just reading and enjoying.
or switch to SCM and just start with lesson 1 and start reading through the books add in some of the BF books if you want to.
SCM is great because it is “enough” but if you decide to add to it there is space for that. For us we have pretty much just used SCM as written with out adding to the books and we have really enjoyed our years, this is out 6th year using the SCM history guides.
Don’t worry about repeating years either. If you finish BF this year and do the same time period next year your kids will be a year older and even if you repeat books (not sure how much cross over of books there are) they will gather new information.