Lots of good advice. Note that the chain ring may not be perfectly centred and the sprocket may not be perfectly circular. At certain points, these factors will work together to create slack spots and tight spots. That is, you think you've set it up correctly, then you turn the cranks and the chain becomes tighter or slacker.

Apart from that, the three pieces of advice I'd give:

1) Finger tighten the nuts first, then adjust the position of the wheel, then nip the nuts tighter alternating between left and right, about 1/4 turn each time — rather than tightening one first then tightening the other.

2) Rather than pulling the wheel back to tighten the chain, "walk" the axle back, a tiny amount on one end, then a tiny amount on the other, and so on.

3) I don't know if this actually helps, but I've always done it as it feels like it should help: always position the spanner so that when you turn it, you are moving the handle towards the rear of the bike. If the bike is on a stand the right way up, that means on the right side, the spanner handle is pointing down, and on the left side the handle is pointing up.

Use proper track nuts with the knurled integral washers. Also, check whether there is any oil or grease on the drop outs/forks that is making slippage more likely.

Good luck.