Want to start your own restaurant in Phang Nga?

Very noble that you want to provide the ladies with a job/income.
There are countless great chefs in Thailand.
From what I read, you already have your plan ready and have planned a 1 or 2 month closure due to the low season.
That basically means you want to target tourists in an area where there is no mass tourism.
But the low season lasts longer than 2 months, so there is a good chance that you will have to cover that period with your assets.
You write that there is no need to make a big profit, but what if you make NO profit at all?

Many people have gone before you, a handful of people manage to survive with a meager profit.
Most people do not succeed and are then forced to go back.
Find a good location, order a drink across the street and spend an afternoon counting how many people pass by.
Next, you estimate how many of those people might choose to buy your product.
Location is important, then you can start making your plan.

There are some nice AI programs, perhaps you can ask your question about a location there, because that was your initial question.
Then you ask that computer about the feasibility of starting a restaurant.

I wish you much success and make sure you don't run out of money.

80% of Thais can make good food.
So with that approach you are doing no more than any other random eatery.

Some places get more customers than others because they are either a nice concept or have something that doesn't exist in that region or because the chef makes a particular dish very tasty.

Robert, beware of laconic responses. After all, you are the one who has to answer questions or not. Then all those from mixed relationships who stock up on kilos of meat at Makro every week and fill the trunk of their pickup truck are really letting it all down. There are those who have nothing to do themselves, but know best how to tackle things. But in the end they don't know the ins and outs. As with many things that play a role in TH. Pilots on shore!
I'm approaching a legal aspect, and you just have to see what you can and want to do with it.

If your Thai woman and her sister start their restaurant, no agency asks them where they finance their business from. Thai 'Tom and Pop' knows that it comes from the farang, because otherwise it wouldn't have been possible.
If they do their weekly shopping and you pay, no official will be there to verify anything.
If your wife and sister-in-law hire extra staff, then there is no problem because they are allowed to do so. A Thai in TH is free and happy in how to shape their own business.

Of course you are the lender. And why an advisor when she has all the experience of running a restaurant. Your role is as I outlined under point 2, and if you want more than that you will have to comply with a lot of formalities. And only one person has to whine who sees you unload the pickup truck every week and get your refrigerators and storage areas in order, and that's it with the fun. Yours, that is. Not mine from a william from Korat.

The website is the only one that makes https://belaws.com/thailand/work-permits-for-foreigners-thai-spouse/ there is a report of it. That would make my previously indicated option 3 easier. Then only B1M company capitalization is needed and the appointment of 2 Thai employees. Having employees requires registration with the SSO and payment of premiums. Robert will have to make clear what he wants to do in his wife's company when applying for the work permit and demonstrate that he is qualified to do so.

But whether one is easier than the other: with my reactions I just want to indicate that a statement like Robert's: "I plan to open a small restaurant with my Thai wife", cannot take place in TH without legal and judicial frameworks.