What should I prepare for my SETV to Thailand and what are my options if I can't get it in time?

Hi all,

Next Monday my partner and I will be traveling from Amsterdam to Bangkok. We have a return ticket BKK-AMS early June 2020. We were hoping to arrange a SETV from the embassy in NL and stay for 60+30 days in Thailand. Afterward, we will probably travel to another country and only come back at the end of our trip (either just to fly out, or to spend the final 2 or so weeks, not sure yet).

Because I was abroad for work and only arrived back yesterday, we are very late with the visa application and are not sure we can still arrange it before Monday. Therefore I was hoping to get some advice to streamline the process and arrange a Plan B if needed:

-I expect our return ticket from BKK in June will not be sufficient and we have to show proof of onward travel for when the visa expires. Should we book onward travel at/around 60 days, or 90 days? I.e., is the 30 days extension always granted, or should we just be safe and book something after 60 days?

-Is proof of leaving the country by bus/air when the visa expires enough, or should we provide a global plan for the entire 8 months? We have been asked to do this when we applied for a Vietnam visa for a previous trip, and it was a pain so I would like to know beforehand.

-Any other documents I should prepare, or prints I should bring?

-If we cannot get the SETV on time, what would be our best option? Visa exemption + 30 days extension, and then try for either another exemption or a SETV from Vietnam/Laos/elsewhere, or is there a better option?

For context, I have been to Thailand almost every year, but always on visa exemption and never extended before. Last trip was October 2018.

Thank you!