Is it safe to assume that USB-A ports are capped at 5V?

There never was a better time to use https://i.imgur.com/fEyWqUsh.jpg

The specification never allowed for more than 5V on USB A (aside from Power Delivery 1.0 but there never was a device using it).

Now, Qualcomm be they damned forever decided to shit on the specification and their chips use a protocol called Quick Charge which does 12V over USB A. However accidentally switching this on is really really hard. See Quick Charge 2.0 Interface in https://www.mouser.com/datasheet/2/328/chiphy_family_datasheet-269468.pdf

This is very , very common in chargers alas.

But https://www.amazon.com/Anker-Charger-Foldable-PowerPort-Samsung/dp/B00VH8ENXE AFAICS does not support anything but 5V.

But you wanted USB C didn't you?

am I not already risking charging damage to the unit?

Well, with USB C, QC or not, the risk is so impossibly low it might as well not exist. If you wanted three 3 C ports, I'd recommend the Minix 100W.