I think I found my own answers.
It's probably as biodegradable as any non-organic natural material. There's no telling what dyes and other chemicals are used to change the basic fabric into clothing but the fabric itself isn't toxic.
https://ecocult.com/rayon-viscose-modal-lyocell-tencel-biodegradable-compostable-microfiber/
The manufacturing on the other hand involves highly toxic chemicals that historically (think within the last five years when they looked) have been allowed to spread throughout the waterways surrounding the manufacturing facilities and have caused serious injuries to the workers handling it.
Forests have been cleared to provide raw materials and to plant monoculture to grow more raw materials. The issues of that alone are so many that I am not getting into it here. Most of us already have an idea.
That's not inevitable, there is growing work both in finding sustainable and renewable sources for raw materials and moving the industry standard manufacturing processes to closed loop systems with less toxic chemicals which protects both the fabric workers and the environment. With careful research and shopping there are times I would be comfortable purchasing something new using some of the related fibers, maybe even feeling like I am helping companies move in a better direction. Those woould be rare circumstances and need much more information than just the word viscose or rayon on the tag.
Second hand I will treat it like a a natural but non-organic fiber.
https://ecocult.com/greenwashing-alert-that-natural-fabric-made-from-plants-might-be-toxic/
If this is an unreliable source or you have more information please feel free to share it with me.