Baking – Biscuits (cookies) turning out more cake-like than biscuity

Your technique is going to be critical here. Leaning scones aren't necessarily indicative of improper technique, but flat ones are.

Keeping your ingredients cold is important when creating scones in every recipe I've read or tried. Cook's Illustrated went so far as to grate the butter and then freeze the grated pieces and use a laminating technique to provide layers. Many other recipes use cold butter (frozen or simply very cold out of the freezer) in chunks and a pastry blender to cut them in. Your milk should also be cold, not room temperature. You may also want to chill your work bowl and utensils. You don't want your butter melting before those scones hit the oven.

When your scones hit the oven and the butter does begin to soften and melt, it will leave behind layers of air in your scone which will help it to rise.

You don't want to handle finished scone dough very much. Use a light touch and minimal work.

I'd also check the date on your self-raising flour. The ingredients will lose their activity over time, causing the flour not to rise as well any more.

this is a technique that might help with the shell issue:

I suspect your croissants are losing humidity during proofing, possibly freezing the butter block, and possibly baking resulting in a harder shell. If you have a humidity controlled proofer and oven, then those parts may not be your problem. Otherwise, read on:

Peter Reinhart in The Bread Baker's Apprentice suggests pouring boiling water in a flat pan/container and placing in the bottom of the oven during the baking (of breads) in order to keep up the humidity. This Link has the recipe published with Peter's permission. Notice how the freezer doesn't come into play (freezer is a dehydrator and pretty good at it, too) although some recipes call the freezing part a must.

Croissants are lighter than bread loafs so they may require a different humidity profile than breads during baking (as noted by commenters). However, for proofing humidity levels should be high and the temperature around 80F or 27C.

To achieve that, for the proofing period keep the dough in a turned-off-oven and turn on the oven light a long with a the boiling water method. This usually prevents 'the skin' formation and a dried out crust. It also helps with getting a better rise during baking. The oven light bumps up the temperature but if you're room temperature is close, don't bother.

You may wish to read this piece on humidity and baking and generally the Maillard Reaction. Here's an excerpt that might explain the problem:

"Under dry oven conditions, the dough will have rapid evaporation of the water from
the exposed surface. This will cause premature formation of a dry inelastic outer shell
on the bread surface"