Will I receive a one-year extension on my Non-OA visa if I enter Thailand before its expiration?

Yes we did it last year on OA visas at the end of our first year. Had a long weekend in Phu Quoc Vietnam (only 55 min flight away) and got stamped in for another year.

We are now coming towards the end of our second year so this time we’ll have to do all the paperwork and go to immigration

If you enter on 18 Sep 2024 with insurance valid until 30 Sep 2025 on non-OA valid until 30 Sep 2024, you will be stamped in until 17 Sep 2025.

Stamped date is earlier one of 365 days or Insurance validity. Not over 365 days (with arrival date is day 1)

No. You'll get a one year stamp from the date of entry, so your new stamp will expire /25

As well there is no financial requirements to activate the upcoming second year. You'd need to bring 800k into a Thai bank at least 2 months before your third year/ first extension is applied for. (If that's your plan) The multiple entry benefit of the original visa will expire at the valid until date , so if you plan to exit Thailand after receiving your permission to stay until stamp you'll need to buy a single or multiple re-entry permit before you depart to keep your permission to stay alive.

If you have an OA visa in its First year of being issued...

If you depart Thailand on or before the expiration date of the OA visa,

You will be stamped in for One added year..

If you have proof of the health insurance required for the OA visa.

The ending date of your health insurance policy dictates the ending date of the stamp you will receive on arrival into Thailand before the of original expiration date of the OA visa.