Without a doubt, if you plan on living in rural Isaan, you will need a car or a truck.
A motorbike is fine for getting around town, however, there will be times where you may need things like western groceries, that are not available for 50+km.
If this is the case, there are no taxis or other public transportation.
I have never heard of Grab working in rural Isaan where I live either.
A car/truck is going to be a necessity.
Any plans to live in rural Isaan should include a car/truck in your budget.
There are tons of solid used cars in the Khon Kaen area, https://visualthailand.com/2026-top-5-used-car-dealers-in-khon-kaen/
You should be able to find a good used car in your price range.