"...your subsistence based on your value (work:life) feels to me like a form of slavery onto itself."

To whom, or what? A frog subsists on it's acquisition of prey no less or more than I do, and everybody does. However, society has developed specialists that raise prey species, produce electricity, roof houses, etc. We have further developed mechanisms of exchange - money - that enable us to predate, build, and empower vicariously by exchanging our specialty work for that of others.

When I earn my pay, to whom am I enslaved?

When I do not, I am enslaved to my paymasters, and will do as they prefer to keep receiving my pay, just as I do the work the community requires to receive my pay for working. This is the danger of UBI, that them dependent on it - and through inflation, everyone will become dependent on it - must grovel to the demands of the paymasters to get it.

That is slavery.

Edit:

"...the utopian side of the coin..."

https://www.sciencehistory.org/stories/magazine/mouse-heaven-or-mouse-hell/