I did actually already do the split you described, and was more referring to how I was supposed to properly address that in the Deductions & Credits section. But I think I figured out where my confusion was.
The phrasing at the top of the page "Don't enter your home office deduction, $X is already included" ends up being very inapplicable in a situation where you claimed a home office for only part of the year.
The reason that TurboTax doesn't seem to handle this page's phrasing well is because it has you split the mortgage interest into the two separate "loans" parts when doing the home office deduction, but the property taxes come in a later home-office deduction section where the split does not exist. And when entering the property taxes in the later section, you therefore have to manually divide yourself based on the months you actually used it, and input that divided amount. But then, TurboTax ports over only this divided amount to the Deductions & Credits section, because it's normally operating in the Deductions & Credits section under the assumption that you operated the home office the entire year / claimed your entire property taxes there; it doesn't realize that the earlier property-tax value is divided, and it thinks it ported over your entire-year property taxes. (It knows the mortgage interest is divided because of the split loans, but not the property tax because that isn't applied at the split-loan level.)
The resolution is to simply adjust the "Property taxes on your main home" field value (in the "Enter any additional property tax paid in 2022" section of Deductions & Credits) to the full amount paid for the full year. (Note that doing this thankfully does not change the divided value back in the home office deduction section.)
On the side note of the 750k limitation, it appears TurboTax calculates that separately later. So I'm not supposed to limit it myself here.
