Horizontal folds pool just under the waistband at the back on everything I make, what adjustment is that actually?

That is a sway back adjustment, and you have the right instinct: it is a known thing with a name, it happens on every pattern, and it is not a size problem.

What it is. Patterns are drafted for a back that is a certain length from waist to hip. If your lower back curves in more than the draft assumes, the pattern's centre back is longer than the distance it has to cover. The extra length has nowhere to go, so it folds. That is why it appears in the same place on everything and why holding the fabric up makes it vanish - you are taking up the excess with your hand.

Why your two attempts made it worse. Taking in the centre back seam removes width, and you do not have too much width, you have too much length. Removing width from centre back also rotates the whole garment, which is exactly the side seams swinging forward that you saw. Going down a size removes width everywhere, which is why the waist got tight.

The fix is to remove length from the centre back only, tapering to nothing at the side seam. On the pattern piece, draw a horizontal line from centre back to side seam a few centimetres below the waist, and fold out a wedge that is your excess at centre back and zero at the side. Everything else keeps its measurements.

Start with a centimetre and a half. Most people need somewhere between one and three, and it is much easier to take out more than to put it back.

The important half of the answer above is where you do it: on the paper, before cutting. Doing it on the garment is possible and it is a worse repair every time.

If you fix it after the fact, you are taking a wedge out of a seam that already has a waistband attached and a zip in it, so you end up with a waistband that no longer matches the garment and a centre back seam with a curve in it that fights the zip. That is the version people try, hate, and conclude the adjustment does not work.

On the paper it is one fold and a redrawn seam line, and everything downstream comes out right.

Two things to watch after the wedge is folded out:

Redraw the centre back seam smooth. Folding out a wedge leaves a step or an angle at centre back. Blend it into a clean line or you will see exactly that angle in the finished garment.

Check the darts. They usually sit in the region you just shortened, and they may now be too long for the space. Shortening them by the same amount you removed keeps the point where it was relative to your body.

Once you have the number, it is yours. Write it on a card and apply it to every pattern you cut from now on, that is the actual payoff, because this is a body thing rather than a pattern thing and it will be the same on all of them.

Front fine, back pooling, side seams straight. That combination has exactly one answer.

A way to measure the amount rather than guessing at it, since "start with a centimetre and a half" is a reasonable opening bid and not an answer.

On a toile, before you adjust anything, draw a horizontal balance line across the back at hip level, parallel to the floor when the pattern is flat. Put the toile on and look at that line in a mirror.

What you will see is the line dipping or rising at centre back rather than staying level. Pin out the fold until the line is horizontal again, and the amount you pinned out at centre back is your adjustment. Measure it, take the toile off, and transfer that number to the pattern.

The reason this beats trial and error is that a fold of fabric is very hard to judge by eye, it looks like the same amount of excess whether it is one centimetre or three, and you cannot see your own back properly anyway. A ruled line is unambiguous and it shows up in a phone photo, which is how you look at your own back honestly when sewing alone.

Same trick works for the front, and for shoulders. Balance lines are the single most useful thing I picked up in fitting, and almost nobody draws them because they look like extra work on a garment you are going to throw away.