How do I keep seed beads from tangling into a mess?

The bunching is almost certainly your tack-down spacing, not the needle. When you string 8 or 9 beads and only anchor every second one, the loose ones between stitches have room to slide and arch, and that arch is what reads as a lump. For a dense cluster I go the other way entirely: string 3 or 4 beads max, then a stitch after every single bead or every bead-and-a-half. More anchoring sounds like more thread on the fabric, but it actually lays flatter because nothing gets a chance to lift. Also keep each stitch snug but not cinched. If you pull to full tension like you would on satin work, the thread drags the beads down into the row below and you get that buckle effect you're describing. Snug means the bead can't slide, not that the thread is taut.

Row direction matters too. If you're curving back over your previous row, come up in the gap between beads rather than through the felt right next to one. Going down through the same hole as an existing stitch compresses everything and is exactly what causes the bulge. Stagger your exit points so each row sits on fresh fabric. On thread: two strands of separated floss is actually a decent call because the strands lie flat and don't fight each other, but floss is soft and that softness is why it kinks. For beadwork I've had much better luck with a dedicated beading thread like Nymo or Fireline. It's thin, doesn't fuzz, and barely twists. If you want to stick with floss, run it over a lump of beeswax once, then run it through your fingers a couple times to work the wax in. That kills most of the twisting. Don't wax beading thread if you switch to it, by the way, it's usually already coated and wax makes it gummy.

One more habit that helped me: every few beads, run your needle down through the fabric and up again nearby with no beads on it. It releases the twist that builds up before it turns into a kink. And if a section starts looking lumpy, don't stitch over it hoping it flattens out. Unpick it while it's small, the thread slips right out if you catch it early.

I messed around with tiny shiny balls on a tote bag once and gave up halfway because the whole patch felt stiff and heavy compared to the rest of the fabric. Reading this makes me think I was probaly doing several things wrong at once. One thing I never figured out is the needle itself, like if the needle is fat it barely fits thru the little hole in each bead, but a skinny needle feels floppy and hard to aim. Is there a trick to picking a needle size that just matches the beads you're using, or is it trial and error every time?

One thing I'd add on the twist problem is thread length, because I learned this the annoying way, that buildup gets noticeably worse the longer your working thread is, so cutting shorter lengths and rethreading more often made a bigger difference for me than anything else I tried. Kind of tedious but it works. Also worth checking wheather your seed beads are actualy uniform, since cheap ones vary in diameter even within the same tube, and a couple of oversized beads mid-row will shove the neighbors around and read as a lump even when your stitch tension is perfect. Do you sort them at all, or just string straight from the tube?

Yeah the snug-not-cinched point is the big one for me too. I ruined a whole cluster on a cuff once by pulling stitches like I was doing satin fill, and the beads popped up in ridges exactly like you're describing. What fixed it was switching to a rocking motion where I hold the bead in place with a thumbnail while I pull, so I can literally feel the moment it stops sliding and back off the tension right there. Sounds fiddly but it takes two seconds per bead. Also worked way better once I stopped trying to build the ball row by row and did it in a spiral from the center out instead.

I'm new here and mostly do cross stitch, so beadwork on a curved shape like yours is way beyond me, but your post made me realize I've been stitching beads straight through my fabric with no felt backing at all and they've always sat kind of wobbly. Is that backing what keeps dense clusters from puckering the whole area? Also, with all that extra anchoring the first reply suggested, do you end up with a mess of thread ends on the back? I never know how to finish off neatly when there are that many stops and starts in one small spot.

Update: tried the shorter thread lengths and it genuinely helped, the kinking basicly stopped. Also strung fewer beads per row and tacked more often, way less buckling. Still wrestling with tension though, my stitches are either too loose or I pull and the beads ridge up. How tight is tight enough, like can the bead still spin a little on its stitch or is that too loose?