Why is my saddle stitch so uneven on thick leather?

At 4.5mm you're fighting two separate problems, and they need different fixes. First is the bunching, which usually means the holes aren't doing enough work for you. On leather that thick, I stopped trusting punch-style irons and switched to marking with a pricking iron, then opening each hole with an awl angled to my stitch. The awl lets the needle slide through instead of forcing the leather apart, and that alone cleaned up my tension massively. Thick veg-tan will distort around a punched hole if the tool isn't perfectly perpendicular, and once a few holes lean, the whole line wanders. Second, skive the seams. Not the visible edges, but where two or more layers stack up before you stitch them. Take the underside of the top layer down by half or so over the width of the seam. You lose nothing structurally because the linen thread is carrying the load, and the leather stops fighting you when you fold and clamp it. If your corners are still stubborn after skiving, dampen them lightly and let them sit under weight overnight before sewing. Trying to persuade stiff leather flat mid-seam is exactly where stitches go crooked. A couple of smaller things that helped me: shorten your stitches on the thick sections. Long stitches look fine on thin lining but they exaggerate any wobble on heavy panels. And wax your thread properly, really drag it across the block a few times, because unwaxed or lightly waxed linen slips and lets stitches loosen between pulls.

Also, get the work clamped in a stitching pony or lace it to the bench edge. Handholding thick panels while pulling tension is nearly impossible, and you won't notice you're tilting the needles until the seam starts drifting. One last thing, don't chase perfect tension by yanking harder. On leather that thick, consistent moderate pulls beat hard pulls every time. Hard yanking is what makes thread bunch at the back side. Same pull, same angle, every stitch.

Something that saved my sanity on my last briefcase: stitch a test strip first. I glue up two offcuts of the same thickness, punch and sew about a dozen stitches on that, and adjust untill it looks right before touching the actual panel. Costs maybe ten minutes and I stopped ruining seams that way. Also, do the long runs in one sitting if you can. I came back to a half-finished gusset seam the next day and the angle I held the needles had drifted enugh that you could see the join halfway down. Same chair, same light, same rhythm, one session. The two week pace is probaly part of the problem more than the technique.

One thing that helped me on thick multi-layer seams was stopping the two-needle-at-once habit. On thin stuff you can pass both needles through the same hole no problem, but on heavy panels I do one needle at a time now and my slants finally stopped drifting. Slower, yeah, but way less swearing.

Also I glue the whole stack up first and let it cure overnight before I even mark holes. If the layers can shift at all while you're working, your line wanders no matter how careful you are. Learned that after redoing a flap seam three times.

Following this one quietly because I only ever work thin, like 2mm card holders and stuff, and I keep putting off my first real bag project because of exactly this fear. I did try a dog collar last year at around that thickness and basially gave up halfway, just glued it and called it a feature. My question is more of a mindset one, do you guys just accept that your first thick project will look a bit rough and treat it as practice, or rip the seam out and redo? I have zero patience for redoing things and it shows.

The issue of keeping large, thick panels truly planar before the needle even touches the hide is often underestimated, and it seems that even with proper clamping, the leather naturally wants to curl, expecially at the edges of the piece. I've found that applying a light coating of leather conditioner, specifically a penetrating oil designed for heavy stock, along the lines of the seam where the panels overlap helps significantly in reducing that inherent stiffness and preventing micro-warping while the thread is being pulled through. Furthermore, if the panels are incredibly large, considering cutting them into manageable, interconnected segments and stitching those segments togather, rather than attempting a single, massive run, can mitigate the compounding stresses and subsequent distortion that you are describing.

Update: fixed it. Turned out I was rushing both needles through the same hole like I do on thin stuff. Went one needle at a time like someone here said and the slant stopped drifting almost immediately. Also did a test strip on two glued offcuts first, which caught my tension being way too tight before I touched the real panels. Seams look proper now.