Possible is a good word here, but I have not heard of anyone on an eligible visa being denied an extension. Some one else may have that story, but I’ve never seen it in this forum.
It is important to note that getting a visa is not the same as being allowed in the country on that visa. The immigration officers have discretion to deny you entry if they believe that your pattern of travel to Thailand is not for tourism in their mind. If they told you that you need to apply for a visa, then do that... But if they told you that this is the last time they will let you in, then you can expect that they have you flagged in the system and will not allow you another entry for some random amount of time. That note in your passport means that you will likely get refused entry and have to purchase a ticket on the spot to either the country you just came from or to your home country... Others here probably have more specific and up to date information on the details of where you have to go when denied entry.
You can get a 30 day extension of a visa waiver... If you have an EU passport, you should have no problem with that. You just need to go to the immigration office nearest to you (Google is your buddy) before your visa expires. Also important to note, that you should not overstay even one day. There is a small fine at the airport for everyday you are over, but if you are caught overstaying anywhere other than the airport on the way out, you will have a really bad day.
Sales tax is included in the membership, but you don't have any kind of income or residence tax based on having the Elite Visa. It is an extended tourist visa. If you buy property you may have some kind of tax for that, but it has nothing to do with the visa or membership in the program.
Last year the KL embassy advised me that they were issuing the last SETV I would get from them because I had too many from them. A few months later, I got a flat out refusal to accept my application for an SETV in Phnom Penh for having too many total SETVs in my passport. It really is a crap shoot now, but Like I stated below, the easiest place I have ever applied for a SETV is in Bali, and they do not have a full page visa. Just a stamp in and out. Good luck.
Bali is super easy to get a SETV and they just have a stamp to get in and out. Flights can be as cheap as 160 USD on Air Asia, but I recommend flying in to BKK and not DMK. A little extra to fly Thai Air or Malaysia Air can save you the additional scrutiny that a lot of people are reporting at DMK.
You can buy a ticket to a neighboring country for less than 50 USD almost any time of the year... You can even get the taxes back if you cancel the ticket. Worth it to not have to deal with the possible hassle, and maybe you will use the ticket and go chill somewhere else for a couple of days...
My most recent Bali SETV was a few months ago, and it was probably my 4th or 5th SETV from that consulate, and my 11th SETV total over the last 3 years. I always had exactly what was listed as required on their site and never had an issue. Hopefully they don't change... Is hands down my favorite SETV experience in the region.
I was thinking about getting an ed visa for 2019 after several years of SETVs, but now I’m just going with the Thai Elite. Keeping up with changing requirements at various embassies and hoping to not get additional scrutiny upon arrival is too much hassle. Until I’m married or 50, it seems to be the best option.