Why do my decorative stitches look fine going one direction and awful going the other?

Almost certainly a feeding issue, not your timing. Here's the logic: if you'd thrown the hook timing off, you'd see it in both directions, because the needle and hook don't care which way the fabric is moving. Directional wonkiness points to how the fabric is being fed, and that's usally something you can fix without cracking the bobbin case open again. A few usual suspects. First, decorative stitches are dense, so they build up thread and pull the fabric as it goes under the foot. On a plain straight seam you never notice it, but on a fancy stitch it shows up as uneven loops. Second, no stabilizer. I know it feels like overkill on a scrap strip, but medium weight cotton without anything behind it will shift around on a dense stitch, and it behaves differently depending on which way the grain and the feed dogs are working together. Even a strip of printer paper under the fabric makes a difference for a test. Third, drag. If part of your scrap is hanging off the front or edge of the machine table in one direction and not the other, that imbalance shows up exactly the way you're describing. Keep the whole piece flat and supported on both sides of the needle, and hold it with the lightest possable guidance rather than pulling it. Another thing worth testing: slow way down on the right to left run. Decorative stitches need the feed dogs to move the fabric a full step between needle drops, and rushing them in your less natural direction is where the unevenness usually creeps in.

For the actual quilts, I'd do what a lot of us end up doing anyway. Sew long border runs all in the same direction where you can, and if a border is too long for one pass, sew from each end toward the middle rather than splitting it left to right versus right to left. Consistent direction plus a light stabilizer behind the decorative runs and you'll get matching results down the whole border.

When I retimed my old machine last year I panicked over the exact same thing, stitches gorgeous one way and scraggly the other. Turned out I was unconsciously steering with my left hand going right to left because I couldn't see the stitch line as well. What fixed it for me was a walking foot. Evened out both directions on long decorative runs and I've never taken it off for border work since. Might be worth borrowing or trying one before you reopen anything. Also curious if you've checked wheather your needle is even slightly burred, mine was doing directional weirdness right before it gave out completly.

Yeah, the walking foot suggestion is the real answer here. I bought mine years ago for stitch-in-the-ditch and figured it was a one-trick gadget, but it ended up living on my machine for any decorative border work. The part nobody tells you is that the even feed changes how the fabric meets the needle, so those dense stitches stop bunching up. I ran a whole set of placemat borders with it last fall and the direction I sewed in stopped mattering entirely. Cheap fix compared to re-timing a machine twice.