Normally, a payment of LLC startup costs is booked in the accounting records of the LLC as an in-kind contribution to the capital of the LLC at its actual cost.
This is not likely to be considered co-mingling if done to the minimum extent necessary. Co-mingling sufficient to impair limited liability protection usually involves using LLC property for personal expenses, rather than using personal money for LLC expenses.
Reporting the expenditures as capital expenditures on an LLC tax form (typically IRS Form 1065) and amortizing it to take some of the expense in the current year will also provide a third-party confirmation subject to auditing of your internal accounting for the expenses.
If you were really worried, you could, after the LLC has been formed, make a contribution to capital to the LLC bank account once you have one, and then cut yourself a reimbursement check for the amounts actually incurred out of the funds made available by the capital contribution against an itemized invoice with supporting information.
The LLC would then amortize the expense of your reimbursement check as allowed under the relevant tax code provisions (historically, these expenses were amortized over fifteen years, but there have been some recent tweaks to that provision allowing shorter term expensing in certain circumstances, so check with your accountant). According to IRS Tax Tip 2021-166:
For costs paid or incurred after September 8, 2008, the business can
deduct a limited amount of start-up and organizational costs. They can
recover the costs they cannot deduct currently over a 180-month
period. This recovery period starts with the month the business begins
to operate active trade or as a business.
A good rule of thumb with respect to co-mingling and related issues is that all transactions of any kind between the LLC and its members should be reflected in clear, contemporaneous, written documentation.