They have insurances with no age limit. I got my insurance here. Expensive to have good insurance at your age, but good agent. Will help you to discuss options.
According to the TAT officials any travel or health insurance from any country that covers medical treatment in Thailand is okay. They have stated so since February/March 2022 (but even last year when Thailand Pass requirements were stricter - my global insurance was accepted for travel).
I have year-round comprehensive global health insurance from French insurance APRIL. I purchased it thought AA insure, broker in Huahin.
Many people are in this position with non-O visa including myself. The duration of insurance doesn't play a role. I always use my year-round health insurance and on one of my recent entries the term date was just about to expire shortly after entry into Thailand. There was no issue at all.