Get the House and Senate to pass it then sign it into law.

If unitary executive theory has any weight he can just execute all the lawmakers and judges that oppose abortion rights.

He would not, because the executive branch does not create laws. He could ask the legislature to pass such a law or to amend the constitution.

"I declare...abortion rights!". Kidding aside, he really needs congress to pass it.

Executive actions that don't need Congress:

• Enforce existing federal healthcare standards such that abortions required for health and safety of mother are included. The Biden DOJ is currently litigating whether emergency rooms must perform care for stillbirths, miscarriages, and life/health threatening pregnancy complications as part of EMTALA.
• Grant support around abortions (but not abortions themselves, because that is forbidden by federal law), such as sick leave or even reimbursable travel expenses for military servicemembers or other federally insured beneficiaries to obtain abortions. This is why Senator Tuberville blocked a bunch of military promotions for almost a year last year.
• Ensure federal licensing for services and drugs that may be used in support of abortions: FDA approval for drugs used in abortions, prescription-by-mail regulations, interstate telemedicine standards, etc. The Biden DOJ is currently litigating several of these issues.

The president also has the veto power for blocking any new federal restrictions on abortion or birth control or IVF.

With Senate approval, Biden can also appoint judges in the judicial vacancies to where there are more abortion-friendly judges in the system.

If Congress gets on board, there can also be legislation for rolling back some federal restrictions on abortion: the federal government runs major health insurance policies (Medicare, Medicaid, Tricare) and actual hospitals/providers (Veterans Affairs, Indian Health Service, military hospitals), which are currently forbidden from administering or insuring abortions. Congress could also guarantee the right to seek telemedicine across state lines, including the right to seek abortion pills by mail.

isn't it a States issue now and not a Federal issue?