As you apparently already knows, no one can give you a straight answer to that question as everything is in the discretion of the IO your standing in front of at the immigration. 180 days seems sometimes to be a turning point, on the other hand you haven’t maximised the METV and stayed 270 days as you could and we also known people have stayed for 270 days just on single entry tourist visa. Personally I would have applied for a 60 days tourist visa before I left, even though you’re entitled to a 60 days visa exemption. Pre approved visa is always a bit “stronger” in the immigration and if something happens you’ll most certainly will manage to enter by a land border with a valid tourist visa. That’s at least the advice I would have given previously.
This is not an official statement, it’s from the Thai Embassy in Oslo Norway, and the embassy in London is equal. Normally an air ticket is your safest bet, but based on feedbacks I know someone is allowed with a bus ticket without any questions, some after some discussion, but some are also denied. If you would go for just a bus ticket I would suggest you printed or saved this from the embassy in Norway and showed to the airline staff if any questions asked.
Now there’s also a direct train from Bangkok to Vientiane.
No documents are needed. Theoretically you could be asked for proof of funds (20K baht) and accommodation (99% sure it wouldn’t happen), but you might risk your airline will ask to see an onward ticket within 60 days.
I don’t have a complete overview but if you check this link from the BOI who will approve your application all options and requirements are well specified.