I haven't crunched numbers or taken a hard look at limitations on M4A or universal multipayer vs our current system, so I've got blind spots. In a political sense, this stuff is hard to communicate effectively from the stump because of cudgel counter-attacks like "death panels!" But does part of having a more balanced, affordable, and available health care system involve capping spending with potentially necessary accompanying changes in the psyche of Americans about what they are entitled to and what constitutes good health care? I can see a hybrid where there is a big boost in universal M4A availability of primary care coverage, scripts, and mental health and chemical dependency treatment, but other medical expenditure domains, i.e end of life care scenarios and long term care for dementia, being covered by choosing separate private insurance to cover those areas. The caveat would be that private add-on insurances would be excluded from covering primary care and other major M4A covered domains. Again, at some level it would likely require a shift thinking about the upper limit on costs every person is entitled to absent add-on private insurance.
The costs of treatment in areas like primary care, mental health, and chemical dependency is an area where costs could be dropped considerably over what the current market bears. Govt could fund its own staff and facilities (non-tertiary care) in those areas pretty efficiently. Covering and expanding access to mental health and CD services in particular seems like the biggest bang for the buck from a standpoint of reducing overall population morbidity, lost workplace productivity, and long term collateral medical costs. They're also the areas where people tend to struggle the most across all ages getting insurance to adequately cover sufficient treatment let alone having timely access.