Always check the stamp while still near the IO. One time I forgot, few days later I saw I was given a 30 day stamp despite having a PE (Elite) and was in the PE immigration line.
I wrote them about 1.5 months before my PE would run out thinking l was gonna have to cough up 1mill. That's when they sent the response that I could just upgrade by paying the difference.
You can also just go for 5 year option to begin with 600k and before it runs out you can pay the 400k diff up to the 1mill and receive another 15 years.
No reason at all to use an agent. Same documents you need to send in both cases. Go for the direct approach with Elite. After sending the required documents there's about 10 days waiting period for their checks. After approval you pay and arrange whether you want the visa affixed in Thailand or when arriving at the airport. You get a welcome package and membership card.
Lots of perks with Elite visa. Free lounge access at the airport. Fast track immigration. None of that onwards ticket nonsense when checking in abroad headed to Thailand. Limo (BMW 730) service driving you home or pickup from home to the airport. No hassle at the bank if you need to transfer money out of Thailand. ( They always ask for work permit first, just flash the membership card). Assistance getting drivers license etc etc
It's super easy whether in Thailand when applying or outside. They are very professional and specific about documents needed ( all via email) takes about a week for them to do checks on you and then you pay. Get your sticker affixed in Suvarnabhumi when you land or at main immigration office at Chang Wattana if already in the country. Go for the 5 year one first, it can be extended for another 15 years for just 400k if you apply before initial 5 years expire.