Your own personal phone has your employers device fleet management software on it?

Yes, because they require it for internal apps. But as "user enrollment" so they are very limited in what they can do. See the table at the bottom of this page[1].

For context this is relatively new and is different from the older way of doing things (device enrollment).

1. https://support.apple.com/guide/deployment/user-enrollment-a...

Does your employer provide a cell phone subsidy? That is how most places do it, they give you some amount of money every month and you sign something that they can enforce policy and seize your device as required.

What level of subsidy is needed for people on six-figure salaries to let their boss MITM their personal web browsing?

Enough to pay for a dedicated work phone & plan so they aren't MITMing your personal web browsing.

Strangely, not very much.