iPAD Pro ONLY charges (slowly) when plugged into a computer

Very few computer USB ports can provide sufficient power to charge an iPad. Most iPad models require at least 10-12W at 5V to reliably charge an iPad.

Many desktop and laptop computer USB ports are typically limited to 2.5-5W at 5V (i.e., 500mA to 1A). Some do have a “high power” USB port, but even these may be inadequate to charge some models of iPad.

Most current iPad models are supplied with either a 12W or 18W Power Adapter.

I hope this information is helpful in explaining the probable cause of the charging behaviour that you observe.

What wattage power/charging adapter blocks do you have/own and are they Apple power.charging blocks or good quality, “Made for IOS devices” good quality, third party power/charging adapter blocks?

Edit: redundant.

At this point, I suggest contacting/reaching out to Apple Product support and services.

Tap on the first link below and scroll down some to get to the Apple Product Support section and phone numbers for your location/region.

https://www.apple.com/contact/

OR

https://support.apple.com/en-us/HT201232

If you need to, if initial 1st tier Apple customer support is of no help to you, you can calmly ask to have this issue escalated up to more responsible, senior level Apple customer support employees to see if they can come up with other possible advice/solutions to your 1st gen iPad Pro charging issues, but I make no guarantees, whatsoever.

If need be, you can have your iPad issues escalated all the way up the chain to Apple engineering level customer phone support.

Even if no acceptable resolution, Apple, AT LEAST, logs all support calls and issues.

Best of Luck to You!

Have you tried plugging in a good quality surge strip into a mains outlet and using all these cables and power adapter blocks plugged into a power strip instead?