Do I need to book my outbound flight to Malaysia and accommodation before applying for a Thailand Electronic Visa?

I had the same question and recently spoke with with a Thai Embassy. They said, their not bothered about the exit date for a visa. However. The airline taking you to Thailand may turn you away at the airport if your itinerary is longer than your visa. It might be advisable to have a flight out before the visa expires... I'm pondering the same situation and nobody knows what will happen at the airport.

You'll need onward travel within 60 days for the Thai TR visa

No You don’t need to book in advance

How long are you planning on spending in Thailand?

There's a lot of options now because of the impending change to visa exempt, so here is my suggestion based on how long you will be in Thailand.

The visa exempt increases to 45 days next month.

45 days or less in Thailand before before going to Malaysia, I wouldn't bother with the visa and just use the 45 day stamp on entry.

45-60 days, then I would get the visa so you can save the time from going to immigration for the extension while you are here.

60-75 days, don't get the visa because you are going to have to go immigration to get an extension anyways, so just extend the 45 day visa exempt stamp at immigration and save a little money.

75-90 days, the only way to stay this long is with tourist visa, so you'll need to get the visa and also get an extension at immigration.

I don't know if the London consulate wants to see your onward travel, I know in the US they wanted to see my flight to Thailand and my flight out of Thailand when I applied for the visa, but each office has their own requirements.

The airline is the ones who usually ask when you are checking in though, so it's best to have your flight out of Thailand ready by then. If you end up applying for TR visa, you can do it without the Malaysia flight and then if they want it they will e-mail you and ask for it.