Both of my access points are only showing 6% free disk space, and my conversely, my hAP ax router has 70+% of free space. Is this "Normal" for the cAP ac access point?

Given cap AC only has 16Mb flash whereas hap AX/AX2/AX3 (which hap AX ??) have 128Mb, yeah, that's normal.

But... large part of that "problem" is because starting 7.13 ROS-package became quite a bit heavier and a lot of those older devices only having 16Mb can get quite tight w.r.t. storage space.
Mikrotik has acknowledged the problem and they are working on slimming down the size again.
How ? When ? Nobody knows.

Anyhow: if your cap ac is only used as access point, there shouldn't be a problem.

When you do a manual upgrade via packages, I assume the APK goes into memory? My client's cAP ac only have about 1MB free storage so I assume this must be the case. Also, I assume the entire RouterOS system is stored on the 16MB storage hence the reason it's getting tight?

Yup, devices with less than 32MB flash and more than 32MB RAM have their "storage root" in RAM. To verify that this is indeed true, check contents of storage root (/file print), if it contains folder "flash", then this scheme is in power. And upgrade packages are always downloaded into this storage root (if using ROS in-built upgrade mechanism, downloaded files are not visible though). Upon installation, files are stored on actual flash.

Regarding RAM disk: my Audience doesn't have it (128MB flash, 256MB RAM), my RB951G doesn't have it (64MB flash, 128MB RAM), my hAP ac2 does have it (15.2MB flash, 256MB RAM), my hAP RB951Ui has it (16MB flash, 64MB RAM), my hAP ac lite has it (16MB flash, 64MB RAM).

Thank you for the technical insight!