Yes, that would absolutely work and is what I was suggesting.
It is probably what I would do if I was starting when you are and know what I know. (having used K and 1st) I do love both K and 1st, but I definitely don't think you need them. In fact, this is basically what we are going to be doing next year. The only difference being that I already own K and 1st of MFW. So, we will use them for our phonics and handwriting. If I didn't own it, I would probably get something that is specifically phonics and handwriting to save money. So, yes, makes perfect sense!
I have not done Adventures. So, what I am telling you comes from reading and researching for next year on their forum. What I have read on the forum about the student sheets in Adventures is that it is a good idea to buy them for all the kids you plan to have tagging along. Your "older" children will do them as written. Your youngers will do some as written and some they will just color on, but it makes them feel like they are definitely part of school. I've seen several threads on this, both people saying how glad they are they got a set for each and others saying they wish they had. I am planning to get them for my two 5 year olds and my 7 year old next fall. I'm not sure if I will get them for the 3.5 year old. It gets expensive.
For math, I do use Singapore starting in K. I just need a little more guided direction than MFW gives for the math in K and 1st. (though, they have a new edition of K and are working on 1st. I believe they have changed the math a little.) I like Singapore, but you definitely can use something else.
Hope this helps! I'm glad you are getting it figured out. Deciding on curriculum is so tough....and fun...and stressful too!