iPad only charges when connected to my Mac

When charging from the USB port of your Mac, you will likely be charging at standard 5V USB bus voltage - limiting the charge current to ~2.4A (12W). By contrast, your USB-C Power Adapters will be using USB PD (Power Delivery).

When using USB PD, the Power Adapter and the charge-controller of connected device (in this case your iPad) mutually negotiate a higher charging voltage, this permitting higher power transfer while keeping the charging current at manageable levels. Higher current requires much larger conductors within cables.

Your description strongly suggests that your iPad's charge controller has a fault, is failing to work with USB PD power sources, but can still accept charge at 5V supplied by your computer.

For practical purposes, in lieu of repairing and returning your own iPad, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair. Unless you have benefit of an active AppleCare plan, an out-of-warranty repair while expensive will be somewhat less than the cost of a replacement iPad.

As for out-of-warranty repair costs, Apple offers a fixed-price price repair. You can verify eligibility and cost for your region/country here:

iPad Repair & Service - Apple Support

If you have an active AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.