Replaced axle on rear wheel. Once I ride it, it tightens and wheel doesn't spin
 

Yesterday, the axle on my rear wheel snapped. I spent today trying to replace it with a new axle of same length. Before doing so, I checked bearings, and they seemed ok. When I put the hardware on the new axle, the axle spins (although feels a bit gritty), but as soon as I ride it, something tightens and the axle no longer spins, and I need to push the bike, dragging the wheel.
What have I done wrong? ?
1? Is it likely there is damage to the bearings/hub that I haven't spotted? ?
2? Perhaps I have put in too many/too few ball bearings? (I didn't count them so have replaced with what looks right). Should they sit snug, some slightly above the others, or all on same level with slight gaps? ?
3? Have I not used the lock nuts appropriately? Perhaps the cones are still spinning?

I'm guessing 3 from the armchair on the other side of the internet.

You probably don't have the lock nut sufficiently snug against the bearing/bearing nut. This is what cone wrenches are for. You can hold the bearing in place with one cone wrench and crank down really hard on the locknut with a second without tightening the bearing.

Hint: a QR Skewer compresses the axle, so bearing adjustments are done in anticipation of that fact.

Thank you ! So even if the lock nuts are on the bearing nuts really tight, it shouldn't affect the ability of the axle to turn? Is that determined by the cones distance from one another/fit to bearings? Or is it important to get lock nuts tight but not too tight?

Is any wear on the inside of the hub a reason to replace the wheel? I can feel faint scratches but nothing that feels significant or deep.

You have to have the lock nut tight enough so that it LOCKS the cone in place and prevents it turning on the axle and tightening up.
Why do you think they call it a lock nut?

Since we are all guessing I am guessing it is a combination of 2 and 3. Fill the race with bearings and remove one, they should not be crowding together so that some are being pushed up. Too-tight adjustment will cause a gritty feel.
If the cone and locknut are not locked together sufficiently tightly the cone can tighten itself during riding. You can use the locking process to fine-tune the bearing adjustment, turning the locknut against the cone tightens the adjustment, turning the cone against the locknut loosens it. Tightening a quick release will compress the axle slightly, tightening the adjustment, so this must be allowed for. The use of an axle vise is very helpful, here is a good one: https://www.steintool.com/portfolio-...ub-axle-vises/
Here are a couple of articles which may be helpful.
https://www.sheldonbrown.com/tooltips/hubs.html
https://www.parktool.com/blog/repair...and-adjustment

FIRST - what kind of bike / wheel?

I’m going for the long game. Shimano hub, loose free hub body.

what do I get if I win?