Do I need to provide an onward flight when applying for a Thai Multiple Entry Tourist Visa from Vietnam?

Thailand Immigration Officers at the border (eg. bangkok airport) have the right to ask you to show an onward journey (if your METV is fixed for -days), but airlines might ask the same question at check-in or they will not permit boarding - because they have to transport you back and maybe fined if not allowed in. Generally, you would only need to show your first intended exit/onward journey. This was required even using the old paper applications and passport visa stickers systems.

You upload a pdf with everything in it.. avoid them asking you again

Typically you show your initial flight into Thailand and a flight out within the time you're initially stamped in for which is 60 days and then a return trip to Thailand sometime after that. Without demonstrating proof of need you're only going to get a single entry tourist visa.

You cannot stay Thailand forever on repeated METV. IM will not accept repeated METVs as a "trick" to stay Thailand without showing larger money.

Just don’t tell them you have a Thai wife and book an exit ticket, TDAC and also at least a day of hotel booking, this should be enough to get the entry

You say you're not interested in a Non O but you need to be careful entering Thailand any other way. Do not tell immigration that you are visiting your wife - you are not classed as a tourist so any visa issued for tourism purposes is the wrong visa. You are unlikely to be denied entry but if you are questioned and admit that you are visiting your wife, you are very likely to be formally warned and told to get a visa next time - if not you will be denied. This is not my opinion, it was told to both me and a friend by an immigration officer at the airport. We had both previously had Non O visas but as multiple entry Non O's got more difficult to obtain we opted to use exempt entries. You'd have to be unlucky for the above to happen but it can so don't say "I'm visiting my wife" if you're asked. If they issue you with a warning, you won't be entering again without a Non O unless you can show them a divorce certificate. I recently obtained a Non O based on marriage and it was very easy to get but I accept that having to show 400k in the bank is a bit much for a short visit - I don't think they thought that through when they stopped issuing Multi Entry Non O's completely. In addition, I didn't have to show any flight bookings or accommodation details. I just gave flight numbers and my home address in Thailand.

One question: Can I apply for a multiple-entry METV visa at any Thai consulate? Not just the Thai consulate in my home country?

They could ask you for proof of leaving if on travel booking confirmation you uploaded one way ticket to Thailand