Did you do the same kind of measuring for the templates for the squares that you did for the sashing? EQ accounts for the 1/4 inch seams. If you add .5 to the sashing, then to me you should have added to the block also. Try a sample block and sashing using the exact measurements EQ gave you and see if that doesn't work.

Well I know that what I would have done was selected the block and done a rotary cutting chart, then followed those instructions. When I do that I select block size from quilt and it shows it as 10x10

10 X 10 finished or unfinished? If it's 10 X 10 finished then your individual blocks would be 10.5 X 10.5 and sashing would be 10.5 X 1.5 before sewing them together, resulting in a 10 X 10 and 10 X 1 after sewing your .25 inch seams. If unfinished it would measure 10 X 10 and 10 X 1 respectively and the block would end up being 9.5 X 9.5 and the sashing would be 9.5 X .5 after sewing them together. You can't change the measurements of one without changing the measurements of all or it will throw it off.

Welcome and good luck. Sometimes if I have a block or several that are just a little off I try them in different positions. Like if I tried it in the upper left and it didn't match but for some reason if I put it in the bottom right it matches. It looks like some of those blocks can be interchangeable so I might try something like that before I start ripping (frogging) out stitches.
A quarter inch foot is a great item to have for in the future. It is a foot the has a guide built on it that you put your fabrics up to as you sew to help keep it in line. Even if your needle doesn't adjust side to side that will help make your seams more consistent.

in piecing a block with so many seams, it is important to test to make sure each part is finishing up the size it should be. At this point, I suggest you measure the block and square them up and then measure to determine how long the sashing should be.
For instance, if they should have finished at 10.5" but didn't, then work with the new size. If they finished at 10" then the sashing should be 10" etc. I hope this is clear.
The posts have indicated what the problem might be - seams not consistent, cutting not consistent etc.
sewing patchwork is only as good as the last seam.
And....don't assume the 1/4" foot is accurate but mark where the seam allowance should be not where the foot says it is.

I think it's beautiful even if it has faults.

I measured the "middle" squares and they are 9.5 in because they've already been sewn to the other squares. So I'm assuming 10" unfinished, which means I made the sashing too long

You might try checking each block with a ruler to make sure they are all right on also.

Yup. Just cut your sashing 10 X 1 and everything should work out.

When I took a workshop from Carol Doak she told us to pin pin and then to machine baste each inter section.
I know this sound crazy but if you get a sample block and you watch the way it feed through the feed dogs you will see some of the fabric doesn't go at the same time.
Try piecing with a walking foot if you don't have a dual feed machine.