I'm also looking and researching into the sponsorship and I have read on an Embassy website, you can have a sponsor, such as a Thai person or a foreigner on an LTV, either must have the 500k and provide a letter advising they will cover you. Some embassies would consider a family member, but discretionary and very much down to the specific one you are applying to. I'm waiting that response.
PS - I'm not an expert and have no clear yes or no. I am just sharing my current findings.
Just wanted to say, doing it on your own, be prepared for a knock back these days. I have a similar visa history as you, went out into Malaysia (ok different location I know) but on return they said no! They saw I was married my visa history and immediately said no....... until I paid 2000baht. Then I got welcomed.
In reply to your question, I had the following experience just a couple days ago, but my situation is slightly different to yours by the sounds of it.
I had a retirement visa here, walked through DanNok border control. Stayed two nights and came back on third day. I actually went for a job interview, even had the invitation letter etc. But timed it to coincide with visa expiration so as to enter on a 60 day to execute the relocation of my Thai wife and I over the coming couple months. (No point buying another LTV as only here short term to move)
Thai Imm on the way out many questions why
Malay Imm - welcome welcome welcome
Malay Imm in - thank you come again
Thai Imm - immediately removed to the office - lady boss no chance to explain said go embassy get visa, managed to explain some, she changed to show me onward bound ticket. I again tried to explain we'd drive over the border. Said no, must show flight e-ticket or no entry.
Stepped outside office - 30 seconds later, man steps up - " you need stamp, give me 2k and I will do now, boss no problem, where you from, ok I get you 60 day."
Done! Back in Thailand. Better for me to use an agent if have to do again. I saw two other foreigners pulled out. Don't know what happened to them.
Lived up in Issan for at least a couple years now, in very remote village locations and the local police and people couldn't be friendlier. Never stopped and always greeted and acknowledged politely. If they've come up to me, it's out of interest and liked to chat with me, often turning to football.