How do I keep my edges from looking all wobbly on a big project?

I just finished a satchel with about the same amount of edge to cover, and yeah, the pooling and thin spots are normal on the first go. What made the difference for me was working in shorter sections, maybe 15cm at a time, and doing a full pass around the whole bag before moving to the next coat. If you try to paint one long edge start to finish, the stuff at the start is already drying by the time you get back around, and that's when you get the patchiness. Load the applicator lightly too. More thin passes beat fewer thick ones, and a thick coat is exactly what pools in the low spots and stays tacky for days. On sanding between coats: yes, but gently. Once a coat is fully dry, a quick once-over with fine grit, like 400 or so on a sanding sponge, knocks off any ridges and gives the next coat somethig to grip. If I skip it I end up with little lumps that get worse every layer. Two or three coats is usally plenty for a nice finish, four if the edge is rough. Also, dampen or burnish the edge before the first coat. Paint over an open fibre edge soaks in and looks uneven no matter how careful you are.

For the strap, 10mm is actually easier than a narrow edge in some ways because the paint has somewhere to sit. I'd glue the layers first, sand the edge flush and smooth, then burnish the heck out of it with an edge slicker and some water, maybe a dab of gum to help. On thick veg tan you can really lean into it and get a hard, glossy edge before any paint goes on. Some people skip paint on straps entirely once the burnish is good, it's a clean look and one less thing to patch.

And yes, bag number one edges looked rough. The flap curve on mine had a wobbly section I had to sand back and redo entirely. It's metres of edge, nobody's first one is perfect.

Agree with the section-by-section approach, it's the only way I got through the flap on my first tote. One thing that helped me on the long curves: I laid the panel flat and rotated the leather under my hand instead of trying to bend my wrist around the curve. Way more even pressure, way fewer thin spots. Also found a scrap of an old t-shirt over my finger worked better than any applicator bottle for feeding a tiny amount of paint along.

One thing I learned the hard way on my first briefcase: do the edge work before you stitch the panels together whenever you can. I fought the assembled bag for days and it was miserable. Now I paint edges flat on the bench, much faster and the coats come out more even. Obviously you can't do that everywhere on a bag, but the flap and any straight runs are fair game. On drying time, I stopped rushing it. I gave each coat overnight near a radiator and the difference was night and day. Rushing the last coat is what turned tacky and smudged for me. Did you stitch through the strap edges or is that 10mm solid stacked leather?

Update: tried the 15cm sections thing and it's a game changer, no more pooling. Also rotating the panel flat on the bench like the second reply said saved my wrist on the flap curve. Three coats and it acutally looks decent. Burnished the strap edges too, way less scary than I thought.