Miscellaneous thoughts:
1. The Rule of 55 is unusual in that you only need to separate from service in the year you turn 55, not after you turn 55. So from a "Rule of 55" perspective, any date in 2025 works.
2. The "healthcare the entire month if you retire in that month" is not true of all companies. It was true of my last employer, but has not been true for others. Check with your HR department.
3. I Roth convert every year instead of every other. I do take ACA subsidy loss into consideration and view it as an additional parallel tax system. I don't think there's much tax benefit in alternating - at least, I haven't seen any arguments for that idea.
4. You might be able to "take" your vacation time at the end of your work career. This may extend your actual retirement date. So if you have 60 days of vacation time and want to leave work on April 2nd, then it may make sense to ask your employer to take those 60 days of vacation time starting around February 3rd. So you last actual day in the office is February 3rd but your health insurance might run through April 30th. This has value if you really want to leave and have stock options that don't vest until April 1st or something like that.
It's been a while, but on the ACA, you probably need to sign up for coverage by the 15th of the month in order to start ACA coverage on the 1st of the following month. So by April 15th in the example dates from the previous paragraph.
You didn't ask, but if you retire early in the year, that might impact your tax planning for 2025 because your income might be relatively low. As an example, I did a Roth conversion in my first year of retirement. I also changed my 401(k) deferral down to just whatever was matched; I didn't actually want the tax deferral because my retirement year was a low income year.