It is relevant that Jefferson died rather deeply in debt. His creditors had a claim on his slaves, so he wasn’t really free to emancipate them.
OTOH, he could have lived a little less lavishly and gotten out of debt so he could free his slaves.
G. Washington unostentatiously not only freed but made financial provision for the care of his slaves in his will.
So you maybe wouldn't want to be Jefferson's slave, but one of Washington's, so that you could be assured that you'd be freed once he died, and your children (what Jefferson called "an addition to capital") wouldn't be sold.
Unfortunately, slavery didn't operate on the basis of slave choosing master.
If he had then by Virginia law those freed slaves would have had 12 months in which to vacate the commonwealth or else they could be sold back into slavery.