I would recommend reading about the founding fathers, including Jefferson. Jefferson wrote extensively against slavery and even included as such in the initial draft of the declaration of independence (without going so far as to make it unlawful) - but that was apparently stripped out by the Continental Congress. In 1784 he attempted to pass legislation federally banning slavery - it failed to pass by one vote.
Of course this might seem hypocritical given he indeed did own a substantial number of slaves, but as today the appeal of convenience often outweighs ideology when systems are widespread. For instance Warren Buffet has argued that a larger tax on the ultra-rich would be socially beneficial and I think he certainly does believe that, yet of course he himself does everything he can to minimize his own tax bill in the mean time. On the consumer level, I doubt any of us really support slavery or systems of near slavery yet it rarely stops us from buying Apple products in the near slavery case, or Hershey/Nestle/etc chocolate in the literal slavery case.
On abolition versus the phasing out issue, Jefferson also wrote there. He wanted to phase out slavery and ensure the slaves were skilled, educated, and had sufficient resources to make it on their own. He felt that abruptly abolishing slavery would lead to violence, both from dependent slave owners who would not go peacefully as well as from slaves themselves. He also thought that abrupt abolition would lead to poor outcomes for the slaves as they would set free in a land driven by the merit of the individual with generally minimal to no education, skills, or ability.
To avoid the system you mention where slave owners would simply entrench themselves by 'banning competition', he even proposed the notion of the US government buying slave children for a nominal fee, training them, and then sending them abroad (to Santo Domingo in particular) to make their way as skilled freemen. Essentially, a way to ease the transition from slavery to freedom for slave owners and slaves alike. One must say such a system feels dystopic, yet on the other hand it's interesting to consider whether such an idea would have had a better social outcome in the longrun. The civil war divides of this nation remain to this very day, as does the inequity of the now long distanced descendents of former slaves.
https://www.britannica.com/topic/The-Founding-Fathers-and-Sl...
https://en.wikipedia.org/wiki/Thomas_Jefferson_and_slavery