Will I be fined if my 90-day report isn't confirmed by the due date due to holidays in Thailand?

up to 7 days after.in person... no problem

Go immigration today or tomorrow and do

There's no grace period due to holidays. Immigration is currently open and there is no holiday until the 29th and the 1st.

You're allowed to file online 14 days before the due date up until the due date. But you can file in person up to 7 days after the due date with no fine. So if your submission is not approved by the 3rd or 4th you can cancel it in the system and then go file your TM47 in person.