I plan to use 1-1/2' conduit and bury a phone line and water line,
all 36" deep while I am at it. The writing on the wire is RW90 XLPE
1000V, which is moisture resistant as far as I can tell from a quick
web search.
snip

Also, this wire insulation is not listed in any of the NEC tables for
use as general wiring. A quick google search shows this cable is used
for transmission/switchgear and some mining use.

Buy the correct cable for the job. Then you can size it for your
needs.

Nate

It's a standard Canadian wire designation, suitable for use in wet
locations. Most service entrance conductors here are RW90 insulated.

Ideally you could sell the single conductor #8 or trade it in for some
larger cable. Unfortunately it's not a very useful or sought after item.

With the number of colors you have you'd be limited to 120 Volt circuits,
unless you have lots of the black. It takes 4 conductors to do a
standard 120/240 subfeed. Parallel runs in this gauge is not acceptable.
The feeding breakers are not designed to hold 2 wires anyway.
As others have pointed out the voltage drop at this distance will
probably limit you to 15-20 amps total load at your shop with the #8.

For this application I would probably recommend #2 TECK breakered at 60
amps, or 1/0 ACWU. These are both armored direct bury cables. That
should be enough to run a medium size welder and lights, etc.
(again I've given Canadian cable designations)

Mike