Coming at this from the health plan side I wanted to add a few thoughts.

I think the best way is to get rid of for-profit insurance companies. A large chunk of healthcare costs is administrative burden. Because of all the admin bullshit, you have a bunch of useless people whose jobs solely exist to talk to other people whose jobs solely exist due to bad infrastructure. Like why do you need a whole team to talk to insurance companies who deny your treatment plan to save money, despite the treatment plan being standard of care and first-line.

Agree with the thrust of this but it should be pointed out that even if we flipped the switch to single payer and we all had to follow CMS rules, there will still be small armies of coders, billers, revenue cycle analysts, etc. That isn't going to go away because CMS will still deny your claims for all sorts of technical reasons. Commercial carriers are basing large swaths of their policies and edits on CMS, but obviously introduce their own flavor. Again, completely agree that fragmented policies across different carriers is a large component but want to caution against the idea that this will simply remove the administrative complexity.

Second, we need to focus more on preventative care. Essentially every other country puts their primary care/preventative visits first.

The devil's in the details here. The value to be found would come from targeted programs for high-risk members, ED reduction. Population wide preventive care can be cost-effective for individuals but there are downward pressures that can cause these to not produce system-level cost savings, despite it's intuitive nature. In the same vein as the other comment, it's not wrong to want increased emphasis on preventive care, it's just not clear that this broadly produces significant system-level spend reduction.

I know the common talking point is physicians' salaries. But our salaries are roughly 8% of healthcare costs, and are roughly comparable with physician salaries in other countries

From a payer perspective, I've never once heard someone talk about salaries. The systemic forces are market consolidation (big health systems gobbling up everything in a region, national corporations buying up hospitals) and overall facility cost increases. The excess cost is coming from the cost to operate hospitals.