How can I effectively plan my return trip from Thailand if I'm unsure about my visa duration?

I see alot talking about the visa part but not the question over the return flight. Technically you are required to have a return ticket to enter but there is no 1 checking and even when I extended this year I wasn't asked for any proof I had a ticket. I got my visa extention and just before my visa ran out I booked a return flight and left the day my visa extention expired

You cannot get a 60 day extension at all on a 30 day visa exempt, you will only get a one off 30 day extension on your 30 day visa exempt stamp.

Get a 60 day Tourist visa before coming via the evisa website. You can extend this by 30 days at immigration in Thailand giving you 90 days total. If you want more time you'll need to leave and re-enter, border run or fly to neighbouring country and back. You can then enter visa exempt and you'll get stamped in for 30 days.

You don't require a return ticket but might need proof of onward travel. You can just buy a ticket to a neighbouring country for around 2000B

First of all, your numbers are not correct.

If you travel without a visa (visa exempt) then will receive a 30 day stamp for entry into Thailand.

You can apply one time for a 30 day extension, not a 60 day extension. That's 60 days total.

If you apply for a tourist visa online before you travel to Thailand then you'll be stamped in for 60 days on arrival and can apply one time for a 30 day extension. That's 90 days in total.

If you want more than 90 days then you would need to leave Thailand and return to get a new entry stamp.

Your airline might not let you board your flight to Thailand without showing a flight out within 30 or 60 days depending on if you have a visa or not so you'll want to check with them to see what their policy is.

you enter 30 days visa exempt and get a 30 day extension = total 60 days or 60 days tourist visa and get 30 days extension = total 90 days