I proposed finding professional help, and a safe person to help ops partner. Not throwing them in the deep end.

You proposed a "clean cut", which is the deep end as far as OP's partner is concerned.

Throwing a scared non-swimmer into the deep end is cruel, dangerous, impatient and unnecessary - even if there is a lifeguard (mental health professional) nearby. Lifeguards should definitely be employed, but they are fallible: they may be busy rescuing others, or asleep, or off the clock.

Building skills in the shallow end first, where self-rescue is manageable, is far safer than relying solely on a lifeguard.

You were doing the best you could at that time.

Not only at that time - at any time. There is no better approach than to be compassionate and patient, instead of heartless and impatient - as long as one does not lose sight of the goal or let oneself be trapped.

I did not lose sight of the goal or let myself be trapped.

Likewise, I advised OP, above, how to set boundaries and establish a clear but compassionate path out of the relationship.

You can't change the past, but we can learn and grow, when we notice mistakes.

What I did was not a mistake. Neither is what I proposed to OP.

ops partner won't be able to rewire their brain without a clean cut

That is almost certainly a false statement. You have no evidence for it. Neural plasticity is far greater than you seem to think, even on the spectrum.

You are proposing op stay and help ops partner, regardless of how they're feeling or coping.

Hardly! I'm saying OP should think of the future - not only the present - and should recognise that a little extra effort and inconvenience in the short term will yield much greater peace of mind in the long term.

Because you did it.

I'm not proposing it because I did it.

I'm proposing it because it is the humane, mature, responsible course.