NEVER do that by yourself. Use a "Visarun Company". Only these guys gurantee a seamless border run on the same day. You seem to have little knowledge of what is going on at the borders at the moment regarding visa-exempt entries. Hundreds of people have been denied entry in the past months. It's exactly those people who had taken the 30 days extension, and are refused a second visa-exempt entry because they try to maxx out their stay in Thailand . . it is all over the FB groups. Beware!
you better DON'T take a 30 days extension on top of your first visa-exempt entry. Avoid to maxx out your first visa-exempt stay. . . . . . . Rather exit before the 60 days stay permit expires, stay in a neighbour country for a week, and come back on a second visa exempt. You will then be able to show proof of the return flight to your home country, as proof of onmward travel, which can be asked by an airline and the Thai Immigration border official. The success is guaranteed. This itinerary is that of a "real" tourist and not of somebody who tries to live in Thailand on touristic entries and their extensions
you better DON'T take the 30 days extension - avoid to maxxout your first visa-exempt stay. . . . . . . Rather exit before the 60 days stay permit expires, stay in a neighbour country for a week, come back on a second visa exempt and be able to show proof of the return flight to your home country. Success guaranteed
it takes as long as it takes. Sometimes you are asked to upload more documents, which can delay the process. Check their website, they published e-visa applications taking between 5 and 10 days. Keep in mind you will need to stay in Vietnam until the DTV gets approved or not. That's what many are complaining about - loosing time and money whilst having to stay abroad and wait
you should be fine. You will probably be questioned, but since you won't maxx out the visa-exempt stay permit for its full extend of currently 60 days, staying only for 21 days before you fly to Malaysia, you should be regarded as a "real" tourist. However, for your own peace of mind, make sure you got two more proofs besides the onward travel ticket: 20,000 THB in Thai Baht or another main currency in CASH (no credit card!), and a proof of a few first nights of a booked accomodation in Thailand