Do I need to inform immigration in Thailand about my plans to leave with a re-entry permit?

At most airports, even small ones they have a counter to buy your re entry permit. I'm in Hatyai. It was easy.

Very easy, you leave the kindom and entry with your re-entry stamp. I did it right now.

You're fine. Remember to fill in the new online tdac form for your return to Thailand, and possibly file a TM30 upon return.

As long as you already have the re-entry permit that is also valid until January 2026, you don't need to do anything. Just leave like normal, you probably won't even see an immigration officer because you'll go through the automated gates. And then when you return make sure they see your re-entry permit and double check your entry stamp after you step away from the booth to make sure you got the correct stamp until your old January 2026 date. The stamp when you return should say "Non-RE" for Non-immigration re-entry.

I would talk to the imigration, better to be safe than sorry.